Disconnect everything not critical. Drives fans front headers etc. Just the bare minimum to boot the system. CPU one stick of RAM and a video output if your mobo doesn't have one.
Did it boot? If yes, add one piece back to the system at a time. If it stopped booting whatever you added last is causing the issue. If it didn't boot, swap to a different stick of ram try again, if it doesn't boot with any of your ram find someone that can loan you a compatible processor or motherboard to swap those out. See if your CPU or motherboard died. Or take it to a computer shop to test the mobo and CPU against their systems. They might do this for free or for a fee.