Computer will only POST sometimes.. and won't detect HDD when it does
 

I was playing a game on my (~2 yr old) computer a while ago when it locked up on me, and then crashed. Since then, when attempting to power on the system, one of 3 things will happen: 1) It will successfully POST, and then give me an "DISK BOOT FAILURE, INSERT SYSTEM DISK AND PRESS ENTER"...