"Hold down power button until it turns off" - diagnose kernel panic? What could be wrong with my MacBook Air laptop displaying "You need to restart your computer. Hold down the power button until it turns off, then press the power button again. 

The same message still appears even when I reboot.
If your Mac Kernel Panics every time you reboot, well that is a problem. Can you boot into safe mode?

Hold the shift key while booting and release when you see the Apple logo on screen. The boot process may take longer than it usually does. Could be a LOT longer. If that works then there is something you installed that runs at startup that is causing the kernel panic.

If that does not work boot into Recovery Mode (Command-R at boot) and reinstall the OS right over top of what you have now. With luck that will fix it. Be careful not to reformat the drive as that will wipe out everything. An OS reinstall should only give you a fresh copy of the operating system and leave your user files alone.