Luckily, all a PC needs to boot into the BIOS is a PSU, a motherboard (with CPU and RAM) connected to a monitor. If the motherboard doesn't have video processor is needed also, but that complicates things.

So disconnect the SSD and the 2 hard drives, and if possible the video card and try again. If that fails, you know that the PSU, the motherboard, the CPU or the RAM is defective. Then replace what's broken.

We recommend to take everything out of the case to prevent short circuiting when experimenting.