My mac won't boot, stuck on loading bar and I can't reinstall OS as it says disk is locked!

Install Lion Through Mavericks from Scratch

Be sure you backup your files.

1. Restart the computer and after the chime press and hold down the COMMAND and R keys until the Apple logo appears. Wait for the Utility Menu to appear.
2. Select Disk Utility from the main menu and click on the Continue button.
3. After Disk Utility loads select the volume (this is the indented entry, usually Macintosh HD) from the side list. Click on the Erase tab in Disk Utility's main window.
4. Set the Format type to Mac OS Extended (Journaled.) Click on the Apply button
5. When the process has completed quit Disk Utility and return to the Utility Menu.
6. SelectReinstall OS X and click on the Continue button.

This will install the version of OS X you had installed.

The Recovery HD should be locked but still bootable. If it is not bootable then the problem is with the drive, but you have not provided sufficient information such as what the exact iMac model you have and what you have tried to do and how you did it. You need to provide the exact wording of any error dialogs you have received. You can try the following if the disk is locked due to permissions being set incorrectly;

Fix Permissions of a Locked Drive

Enter the following commands at the prompt. Press RETURN after each.

chflags 0 /Volumes/*

chmod a+rx /Volumes/*

You will need to boot from the Recovery HD to perform the above since you cannot start OS X.

Boot to the Recovery HD::

1. Restart the computer.

2. Immediately after the chime press and hold down the

"Command" and "R" keys together.

3. Release the keys when the Utilities Menu appears.

4. From the Utilities menu in the menubar select Terminal.

5. Enter the above lines in Terminal.

In the first effort I assume your drive appeared in Disk Utility's list? In the second were you able to boot from the Recovery HD? If you were, then do this:

Install El Capitan from Scratch

Restart the computer. Immediately after the chime hold down the Command and R keys until the Apple logo appears. When the Utility Menu appears:

1. Select Disk Utility from the Utility Menu and click on Continue button.
2. When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the side list.
3. Click on the Erase icon in Disk Utility's main window. A panel will drop down.
4. Set the Format type to Mac OS Extended (Journaled.)
5. Click on the Apply button, then wait for the Done button to activate and click on it.
6. Quit Disk Utility and return to the Utility Menu.
7. Select Reinstall OS X and click on the Continue button.

Please read what I posted previously.

1. Did the drive appear in the Disk Utility list?
2. Were you able to boot from the Recovery HD?
3. Did you try installing OS X from scratch?

If you are not going to read what I post and repeat what you've posted, then I am not going to be able to help.

DO NOT change the permissions of anything. DO NOT try to reinstall OS X.

The startup drive is failing, or there is some other internal hardware fault.

Back up all data on the drive immediately if you don't already have a current backup. There are ways to back up a computer that isn't fully functional—ask if you need guidance.

Make a "Genius" appointment at an Apple Store, or go to another authorized service provider.

If privacy is a concern, erase the data partition(s) with the option to write zeros* (do this only if you have at least two complete, independent backups, and you know how to restore to an empty drive from any of them.) Don’t erase the recovery partition, if present.

Keeping your confidential data secure during hardware repair

Apple also recommends that you deauthorize a device in the iTunes Store before having it serviced.

*An SSD doesn't need to be zeroed.

Your drive has failed or is failing, most likely. Or there is some other hardware failure. At this point you want to make an appointment at the Apple Retail Store - Genius Bar and take it in for service. You can also look for an Apple Store - Find a Store - Apple.

If it is just the hard drive, then you can try booting the computer from an external drive to be sure the computer is working.

Yes, that is correct. Keep in mind that once you open the iMac and replace the drive you may experience the runaway fan issue because the new drive does not have the temperature sensor needed. I know that OWC does carry a kit which can be used. I don't know other dealers that carry it, so you may have to shop around on Amazon or eBay for one. Or you can use a third-party utility that controls the fan separately. I've found that Macs Fan Control is a good choice having used it on an older iMac I have. It is freeware.

I don't know how old your iMac is but once you open the case and install non-Apple components Apple will no longer provide any support. If you have an active warranty it will be void.

i use this rule when my friend mac has such problem , and it was helpful , thanx