[dba-Tech] Windows won't reboot

My son's friend removed his hard drive to take it home and download some files. He put it back, but now it's acting like there's a disk in the floppy drive and of course there isn't. I suggested he start it in safe mode to see if that would reset it. If he can get it started using Safe Mode, is there a setting he can reset to fix the problem? 

Susan H.