Build your system outside of the case to rule out an electrical short. If you've not done this before, follow this process:
- Get cardboard, wood, or any non-conductive surface
- Disconnect all cables/wires from motherboard, except for CPU fan
- Move any components that might hinder removal of the motherboard from the case
- Remove the motherboard from the case (leave the RAM and CPU installed), then place it on the surface from step 1
- Remove the PSU from the case
- Reinstall your GPU
- Connect the PSU to the GPU and motherboard
- Look on your motherboard for the power switch pins
- Jump the pins with a metal object, like a flathead screwdriver, to power up your rig
- test for stability (if successful)