Q: i need help with startup after changing username.

How did you change your User name?

You can not change an account name in System Preferences; you can add or remove accounts and change a password but changing the actual name requires a lot of technical work via the Terminal app.

Are you trying to log into the computer using an Apple ID instead of a normal log in?

When you enter Safe Mode, can you log into the computer?

If you changed your Apple ID or password for the Apple ID, you may need to reset the Apple ID password.

Ok I understand now. If you change the name on the user account without changing the name of your Home folder, there can be problems with the user account. All of your files are set to work with your original account name, and will not work with your new name unless the name change was done correctly.

Did you change this information while logged into the account you were changing? There may be a way to fix this but it would require another Admin account on the computer. Is there another Admin account on the computer?

Alright then we will have to create a new Admin user account so we can change your account information back to the way it was. Once things are (hopefully) back to normal, I can share the Apple provided steps for changing your account information to ensure no problems with opening files.

To create a new Admin account without access to System Preferences, we have to trick the computer into thinking we are setting it up as a new computer. Don't worry, this won't alter your files or your user, it simply creates a new Admin account that we can use to fix your original account.

Follow these steps exactly or your computer could be damaged.

I have tested this method for creating a new account on my own computer and know it works.

Turn the computer off, then:

1. Turn the computer on and immediately hold "Command-S" on the keyboard (This is Single User Mode) You should see white words on a black screen
2. Mount your hard drive by typing this: /sbin/mount -uw /
   1. Note there is a space between "mount" and "-uw" and also a space between "-uw" and the /
3. Press Enter/Return (nothing appears to happen and this is normal)
4. Now type this: rm /var/db/.applesetupdone
5. Press Enter/Return again
   
   1. Again note the space between "rm" and "/var/db/.applesetupdone"
   2. This removes the file that prevents your computer from running the initial set up
6. Now type "reboot" and press Enter/Return

The computer should boot up as if you have never set it up before. Walk through the initial set up, but ignore anything that asks for an Apple ID or transferring data. We are just making a "dummy" account so we can log in and change your information back to the way it was. We will delete this account in the future unless you really want to keep it.

Give it an easy name and password that you will remember. Again, you are most likely not going to be using this account ever again so don't worry about setting it up with your name or anything else. Just skip through until it loads a new user.

From here, open System Preferences > Users and Groups > Unlock with the password you just created for the dummy account > Control click YOUR account and open the Advanced Settings. Change the Account name and Full name back to the EXACT name you had before. There should not be any spaces unless they were there originally. Also ensure that the Home directory is "/Users/(youraccountname)," where "youraccountname" is the same exact name as the Account name. For example, if your account name is johnsmith, you directory should be "/Users/johnsmith"

My hope is that by putting everything back to the way it was, your computer will recognize the information and allow you access to your account.

Once you verify everything looks the same as before, click OK and then lock the lock. Log out of the dummy account and see if you can log into your original account.

Let me know how it goes.

I am so glad you were able to get this resolved! It's always scary for a user when their data could be in jeopardy. So all is good on your end now? You are able to access you files correctly?

Make sure you mark this as solved in case someone else is looking for a similar solution.

Also, if you want to change the names so you aren't seeing your old name, use the article below. Read through it before do anything to make sure you understand how it works. Changing the Full name is not a problem, it's when you change the Account name and home folder directory.

Change the account name and home directory name on your Mac - Apple Support