My computer was in sleep mode when I unplugged the power cord from it to work on another machine.
But after plugging it back in, it got stuck on a black screen (with prompt) while booting. I reset it and it is now going into an endless reboot loop.
If booting into safe mode as suggested by @KCotreau works, then you should be able to shut down the computer from safe mode and then reboot normally. Often booting once into Safe Mode will fix startup problems. If you can't boot into safe mode, try using the "Repair your computer" option on this menu or "Startup Repair" on the Windows boot CD (Windows 7 / Vista only).
If the normal boot problem continues after successfully booting into Safe mode, then you likely have a problem with corrupted software and/or device drivers. Options vary based on the exact version of Windows you are running, but generally rolling back your system to an earlier date using System Restore or uninstalling any recently installed programs or devices will help.
Try hitting F8 (a few times) right before, and as Windows starts. It should bring you to the screen in the screenshot. Then choose "Last Known Good Configuration", pick your profile (probably only one), and see if that saves you.
Note: in the screenshot, it is picking a different option, arrow down until you get to the right one.
