Shut down the PC, unplug the power cord and hold the power button in for 15 seconds. (if it is the old AT PSU you do not have to hold it in, just click it in wait 15 seconds then click it out)

Open the case and remove the cmos battery for 15 Minutes (note the polarity of the battery before removal) Reinsert the battery after 15 Minutes, close the case and plug it in, See if the post completes, you may have to enter the bios and make some minor changes after clearing the cmos.