Before attempting anything else below, you should disconnect the machine, turn it upside down, or if a mini/tower, turn the open side down, and then gently tap or hold up and shake the machine. Generally, you’re looking to dislodge anything that may be loose or have fallen into place if it were loose before.
Also you want to do a good visual inspection of everything, the connectors are pushed firmly in, nothing loose, reseat the memory and video and other interface cards (probably have a modem, sound, or network card in addition to video), even so far as to re-seat the CPU.
Then try powering up… and then forcing beep codes as suggested below…
To try to force the pc to give you beep codes, assuming the pc still has a working speaker.
With the power disconnected, remove the ram. (use standard static precautions)
plugin/turn on the pc … you should get beeps from the PC Speaker … if memory serves, it would be continuous on/off beep pulse … beep beep beep etc..
if that happens, then put the ram back in, and try booting again. If that doesn’t work, try putting the ram back but removing the video card …
if the beep occurs for video, it would be a short beep set. Video is long beep, then 3 or 4 quick beeps and nothing more.
Mememory beeping would continue so long as the machine is on.
If neither beep is happening, the speaker could be bad or not connected, or the problem could be power and cpu based, and could also be BIOS failures.
Power problems don’t just come from the power supply itself. Depending on the age and conditions and quality of parts.. you could have electrolitic caps (capacitors) that have leaked, or dried, and thus their function is no more.
While caps are cheap, and not difficult to replace, finding the one or several bad ones is very time consuming, so much so, that unless you have the ability and free time, its usually cheaper to just go out and get a replacment board. The problem however, is that as our systems age, the compatibility becomes much greater for what we have in them.
you can get a cheap PC board in the 50 dollar range .. but it uses newer intel or amd cpu’s, the ram may not be compatible, etc..
good luck !!