My iMac created an 'other' user and I can't log in

The following probably won't work if the drive is corrupted as I suspect it is. It also requires compatible Mac hardware and you did not specify your particular model Mac.

If these steps do not appear as written, then it's not an option. In that case skip to the procedure below the horizontal line to attempt the creation of a new User Account with Administrator privileges.

In light of the symptoms you describe, the disk is probably in need of replacement at your earliest opportunity anyway. Restoring its contents will require your Time Machine backup. Leave it disconnected for now.

1. Boot macOS Recovery.
2. At the Mac OS Utilities screen, select "Restore From Time Machine Backup" > Continue. A Restore from Time Machine screen will appear. Read it then Continue.
3. When the Select a Restore Source screen appears, select the internal disk. Its default name is "Macintosh HD" but you may have changed it. Continue.
4. The Select a Local Snapshot screen appears next. Restore From > Local Snapshots on Macintosh HD (or whatever you named its startup disk) will appear in the dropdown menu selection.
5. If a Local Snapshot Date & Time list populates, select the most recent one. Continue.
6. A confirmation dialog will appear. Read it then Continue.
7. A Restoring screen will appear next. That operation won't take long.
8. A Success screen will appear next, and the Mac will restart.

If that doesn't work, continue below.

Read the lengthy procedure that follows. You should probably print it vs. writing down the steps or trying to repeat it from memory. Don't be put off by the length of the instructions. Some of the steps are optional. It's simple and will only take a few moments.

It is also overly conservative, but I have used it myself recently and I know it will work.

As I understand it, you have a Mac with no Administrator accounts. Of course that is not supposed to be possible, but if that really is the case you can use the following technique to create a new, temporary Administrator account, the sole purpose of which will be to log in as an Administrator that can give your normal account Admin privileges.

There may be other techniques to recover from the "impossible" circumstance in which you find yourself, but the following is one that I have successfully used in the past.

Please read everything that follows before continuing. If you have only the one computer you will need to print this for reference.

Prerequisites:

• If you have not done so already, create a reliable backup in the event something unexpected occurs.

  To learn how to use Time Machine please read Use Time Machine to back up or restore your Mac - Apple Support.

• If you configured an EFI Firmware Passwordthat will preclude recovery, unless you know that password.

  A Firmware Password precludes starting a Mac in single-user Mode, so you will need to temporarily remove it before continuing.

• If you encrypted your startup volume with FileVault, its password will also be required to unlock the disk.

  That password may or may not be the same login password you have been accustomed to using.

Confirm each of the above prerequisites before continuing.

Then,

• Power on or restart your Mac.
• At the chime or grey screen, hold ⌘ and S on your keyboard (two fingers) to enter single-user mode.
• At the localhost:/ root# prompt, type

fsck -fy

...and press Return.

This is a simple check for file system integrity and is optional. It may take a few minutes to complete during which time various messages will appear. None of them are relevant unless they indicate some unrecoverable error. Be patient. If you get concerned that the system has stalled or become unresponsive press the Return key. Nothing will happen other than to echo the Return character, advancing the text on the screen, confirming your Mac has not completely frozen.

When the integrity check completes pressing the Return key will result in the localhost prompt again, waiting for your input.

At the localhost:/ root# prompt, type each of the following lines, exactly as written, including capitalization, one line at a time, each line followed by the Return key. There is a single space preceding the first "slash" ( / ) character in each line:

mount -uw /

rm /var/db/.AppleSetupDone

reboot

The Mac will restart, and then take you through the entire setup and registration process that you have not seen since you originally unboxed it. Do not be concerned—none of the above deletes any information. All your pre-existing user accounts will still available, assuming they were intact prior to beginning this procedure.

Do not elect to transfer your information from another Mac: When the "Transfer Information to This Mac" screen appears, select "Do not transfer any information now" and press Continue.

At the "Create a Computer Account" screen, create the new, temporary account using a different "Full name" and "Account name" than the one you already use. Remember the password you select. Provide a password hint if you wish. You don't have to sign in to iCloud or anything else you might decide to do if you wanted to use that User Account for anything else.

"Allow this Account to unlock the disk" will be selected by default, so confirm that it is because that's the key to fixing the problem.

When it completes, log in under that new account. Use System Preferences to change your normal account to "Allow user to administer this computer". Log out, log in under your normal account and verify you can use it without restriction.

After that, you can safely delete the temporary account you just created by following these instructions: Delete a user or group - Apple Support. Before removing it, confirm you don't need any of the files you might have created in that Account.

Do you have current backups?

How to Create a New User Account if You Cannot Access Your Admin Account

Reboot the computer. At the chime press and hold down the COMMAND-S keys to start into Single-user Mode. You will boot to a black screen with scrolling white text, release the keys and wait until it finishes. Enter the following lines at the command prompt pressing RETURN after each:

mount -uw /

rm /var/db/.applesetupdone

reboot

After you reboot the computer you will be taken to the Setup Assistant. When you get to the point where you are asked to set up your new user account, create a new admin account. Be sure to name this user account something different than the admin user account that already exists on the system. When you finish the Setup Assistant, it will automatically log you into the new account.

You can transfer your files from the old account to the new one. See Transferring files from one User Account to another. Once you have moved all your files into your new account, you can delete the old account using Accounts or Users & Groups preferences.

Once you can get logged into your computer you should create a second account for the other user. Restore each user's data from your backups.

Even though it's only offering "Other" with a blank name and password field, try typing in your "short user name" and its password in them.

The "short user name" is the name of that account's Home folder.

There are some other options so don't give up yet.

If they are so ill-equipped they cannot even perform such a simple repair, I am skeptical of anything else they told you.

It's quite likely that Mac's hard disk drive has become corrupted, leaving its backup your only option for recovery. Did you create one?

Yes. It will be especially true if you find that the disk is bad and must be replaced. In that case, you will lose everything unless you are willing to pay a lot to a data recovery service - and they offer no guarantee.

Corrupted and mal-functioning are two different things. The disk may have tested well at their shop. But since then the drive could become corrupted or some system files have been corrupted. There may be no indication that the drive is mechanically bad.