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.