or possible reason
1) hard disk cable (IDE/SATA) (maybe not plug in probably)
2) Power supply provide insufficient power (maybe too old)
3) hard disk (going to die...backup the data for safty reason)
What hard disk is it? SATA?
SATA consume more power than IDE, try unplug some unnecessary components and check if it behavious the same
Make sure the cable plug in probably (to hard disk and to the main board)
borrow a power supply from your friend and try for 1 to 2 days