This might be a good time to go back to fundamentals... Sometimes you need to go back to the most basic setup and try things, then add more as things work.
First, it would be helpful if you could tell us the brand and model of computer... or if a homebuilt, the brand and number of the motherboard.
Some of these questions will be irritating, but will be helpful in getting you up and running.
1. Do you have a floppy drive in that computer? If not, skip this.
If so, we want to boot to a floppy disk. Do you have access to an MS-Dos boot floppy or a Windows 98 Boot disk, or a Windows ME floppy? Ask your friends. Usually somebody has an old floppy boot laying around in a drawer or case.
2. Disconnect the hard drive. Disconnect all memory modules, while wearing rubber gloves or plastic sandwich bags.
3. Do you have on-board video graphics, or do you have a video graphics card? You want some sort of a video graphics connection. Remove any other cards.... sound, modem, network cards.
4. Insert one memory module.
5. Now you should be down to a floppy disc drive, power supply, video graphics connection for your monitor, CPU fan, and one memory module.
6. Depress the on button. Does the light come on the front bezel?
7. Does the power supply fan begin turning. If not, does it turn 1/4 to 1/2 revolution then quit?
8. If the power supply comes on, do you get any kind of image on the screen? Does it attempt to boot to the floppy disk. Does the floppy disk light come on.
9. If it boots to the floppy and you have an image on the screen, we are part way home. Otherwise, we have more work to do.
10. If it remains dead when you depress the power switch, and the fan does not revolve more than one turn inside the power supply, your computer could be dead, or the power supply is dead, or the cpu fan is dead, or the CMOS battery is dead.
11. If the power supply fan turns but nothing else happens, we at least know we are close to fixing it. Change to another memory module and perform the above steps once more.
12. The idea will be to keep changing out components until it will boot reliably to a floppy disk with a viewable image on the screen.
13. Now get back to us... we are now ready to fix it or toss it.