Hi nerdy concepts
1) Your post suggests that computer was running OK and just suddenly developed the fault in question. Could you clarify this?
2) Have you recently added any new hardware or software just prior to when fault appeared?
3) Hopefuly something is dislodged. To start remove the memory module\s, all pci cards and video card, following good ESD practice with power unplugged completely. Try not to touch gold contacts. Blow any dust from them etc including the slots.
Recheck all connections like hard drive cables and power plugs, Tower connections, CPU power cable etc etc etc, also remove any external devices like printer, network cables etc.
Put the video card back in (unless you have an integrated one on your motherboard, if so leave the card out for now and use the onboard one) and one stick of memory to start with, thats all the cards you need. Then try system again.
No point in putting any of the additional cards back in system (sound, LAN, modem etc etc etc) until system is working or not as the case may be. If you do get it working, put back in one at a time, checking system after each one to help find a culprit.
4)If that does not work and you know how to remove the CPU and reseat, do that. You will neeed some thermal paste though before thinking about that.