You don't say how old it is
Try: Different power supply
Remove and replace the CPU
Unplug everything (HDD, optical drive, USB connectors, extra fans) except the mobo power plugs
Is it clean (free from dust)? I fixed a 8 year old PC last week by removing the memory and CPU and going over it and the PSU and board with an industrial blower. Clouds came out for several seconds. It came back to life after that. Of course that might have just been luck