You might be experiencing one of these two issues; either your computer might be restarting constantly or you might be stuck on a Loading Bar!
Rarely, your Mac might experience a software or hardware issue that requires it to restart. When it starts up again, you might see a message that your computer was restarted because of a problem.
Unexpected restarts are usually caused by software installed on your Mac, or by devices connected to your Mac. If the issue causes your Mac to restart every time it attempts to start up, your Mac might eventually shut down. Use the steps at Apple's Documentation for this issue: Link
Note: If you reach the "Reinstall MacOS" step, STOP HERE. Connect with IMT since we will need to backup your data and reinstall the operating system for you. Failure to do this may result in loss of data and a non-functional computer.
Stuck On Loading Bar
If your computer is stuck on a loading bar and shows no sign of progress for more than 45 minutes, then please try the following: Press and hold your "fn", "shift" and "esc" key for 10 seconds. The computer will make a terrible noise. Following this, let go and press "esc" one more time. After logging in, run Software Updates on your computer.
If the above does not work, you will need to connect with IMT to get your computer fixed. We will need to do a full backup and re-image of the computer to fix the issue. In the meantime you can get the computer to boot by booting it to Safe Mode. Please understand that running a computer in Safe Mode is only a temporary workaround as the system will run slowly and in a limited fashion; the only fix is a full re-image.