My Presario F500 doesnt power on.. whats could be

do it only if u r a professional..
assuming that the system bios hs not been flashed or updated in a wrong way..if so u gotta replace the m/b..
1st see if the ram(s) is loose/faulty..open n resit it..
check for ac main supply and fuse..next check all the outputs of smps viz. 0v (black~ground); +12v (yellow); +5v (red, green~ps-on, violet~pg & grey~sb); -12v (blue); -5v(white); +3.3v(orange)..if faulty try repairing or replacing the power supply with a standby n see..
if all above are ok..check the cmos/system battery..(above +3v)..
check the cpu fan rotating and the cpu generating heat..try replacing the cpu with a standby (if no heat is generated)..
there may b a chance for m/b's crystal clock generator to b faulty..try repairing or replacing the m/b with a standby..
hope it'll b enough to solve ur problem..
seek professional assistance of ur system admin/vendor....