Could be many things. You could try the drive in another computer to see if your data is still intact. Or boot off a CD (Linux ISO or something like a bootable utility CD) check the boot order in the system BIOS (if you can get into the BIOS).
You could also have a combination of issues such as corrupted data on the drive AND a defective power supply or motherboard issue. Personally I would pull the drive and verify if it's okay (take it to a tech if you have to and ask if they can access the drive), then start troubleshooting the PC.