hi
this can be due to a number of reasons,
first check the system bios by pressing the f2 key
there check if all the drives and hdd are listed and on . the drive details should not say unknown device
incase it does , then disconnect the drive and restart and hit f1
,
in bios , clear the event log and then reset the bios ,
then save changes and exit
then check the 4 leds on the system tower and let me know what is the led status ,
also if the system is beeping and giving some error with the f1 f2 message