PC hard drive doesn’t work after left off for six monthes

Left off a PC for six monthes, having a dual core new mobile one.
This anterior PC Pentium III 800 mhz shows motherboard red led lightened when I light it, but nothing else happens. No noise of anything neither. Can this be power supply or hard disk cause? After checking it, I first replaced 3v battery on motherboard. But nothing happens, though last time I used some half year before all worked ok. Can someone tell me something ?
Thanks a lot.