I have seen this once. During windows updates the splash screen says do not turn off or disconnect while updating.
Well, my electricity failed (I live in a rural area) and when I went to turn it back on it got stuck in a loop rebooting.This is what I did. I was able to get the DVD/CD drive to open and I put the windows install CD in the drive and got the drive to close.
During the boot screen there is a splash window that briefly shows ,I pressed f12. When I pressed f12 and got a boot sequence screen, then selected it to boot from cd/dvd drive. It booted from cd and a window came up asking what to do - repair windows, reinstall windows, windows restore. I selected windows restore point using my last known good one. Granted, after I was done, I had to go back to windows updates and install them one at a time, instead of all at once. After a required restart to perform windows updates, my machine started up normally.
To be on the safe side, I booted into safe mode (F8) and ran antivirus and malware scans of the hard drive. Nothing was found, so I assumed the computer basically locked into a reboot loop due to conflicts in windows updates. I have not had it happen since. Hope this helps make sure you know what F key to press to get to boot from the CD/DVD drive. I spent about two hours getting it all done.
So my comp inexplicably entered a boot loop. I was attempting to turn it off last night when it, instead of turning off properly, just faded to black and kept running. So I turned it off by holding the power button, and tried to turn it on again for good measure. The fans started up, but no image appeared on the monitor, and it powered down within 10 seconds of starting. After powering down, it repeatedly boots and shuts down. It's a brand new custom built computer running windows 7 ultimate. I'll post the specs if they would help. What other info would you need? The parts should all still be under warranty. Please help me out guys.