The computer will not be able to boot into Windows, it will usually keep reverting to the “Launch Startup Repair” screen. Note that actually running startup repair rarely fixes the problem.
Before trying anything else, see if selecting “Start Windows Normally” works, you would be surprised how often this does.
Shut down the computer and try tapping F8 on starting it back up to attempt accessing safe mode. If you can get into Safe Mode, the problem is going to be a software causing problems on startup of the OS.
If that doesn’t work, Try tapping F12 on startup to attempt to run Hardware Diagnostics (or whichever key depending on the manufacturer). If the HDD passes it’s test, this narrows it down to an OS issue – which is most common.
If neither of the above work, the computer will most likely need to be re-imaged or the error needs to be researched further.