Is there a speaker connected to the motherboard and does it beep when you turn the machine on? If not, you might have a dead CPU or a short circuit somewhere.
Disconnect everything from the motherboard: HDD, FDD, even the graphics card, keyboard and any other devices. Just let it have power and the internal speaker. Without a graphics card, the motherboard should beep like crazy when it is turned on.
When you hear a speaker beep, switch it off, add the graphics card and see whether there is a signal.
If it does not beep, check whether there is a removable CMOS battery on the motherboard. Remove it for a few minutes, then put it back on. On an older system, this could cause the system to go back to factory default settings.
Many motherboards have a RESET jumper that will do the same. If this motherboard has something like this, put a jumper on it, switch on the machine, wait for it to scream, switch it off, remove the jumper, then start it again.
In any case it is a good idea to check all jumper settings on the board: Maybe somebody screwed up the settings.
Once you get a first life signal from the motherboard, add one component after another, not all at once. Switch the box off, connect the device, turn the machine on and see if it still works. Turn it off again, add the next device, power up the system and see if it still works. But start with just the motherboard and an internal speaker connected to the board.
Disconnect everything from the motherboard: HDD, FDD, even the graphics card, keyboard and any other devices. Just let it have power and the internal speaker. Without a graphics card, the motherboard should beep like crazy when it is turned on.
When you hear a speaker beep, switch it off, add the graphics card and see whether there is a signal.
If it does not beep, check whether there is a removable CMOS battery on the motherboard. Remove it for a few minutes, then put it back on. On an older system, this could cause the system to go back to factory default settings.
Many motherboards have a RESET jumper that will do the same. If this motherboard has something like this, put a jumper on it, switch on the machine, wait for it to scream, switch it off, remove the jumper, then start it again.
In any case it is a good idea to check all jumper settings on the board: Maybe somebody screwed up the settings.
Once you get a first life signal from the motherboard, add one component after another, not all at once. Switch the box off, connect the device, turn the machine on and see if it still works. Turn it off again, add the next device, power up the system and see if it still works. But start with just the motherboard and an internal speaker connected to the board.