You don't need a case, drives, or much else for a power up test. If you start small such as PSU, motherboard+CPU+HSF and one stick of ram you usually get a power up. If not, the problem is in those parts.
Again, no case required. Just some cardboard to set the parts on.
Built my computer about 2 years ago and have had zero issues with it. Had the computer on doing nothing when it turned off for no reason. Hit power button to turn back on and PC blinks on for fraction of a second and shuts down. There is no beep. It does this repeatedly. I have replaced the PSU, motherboard, CPU and switched out the ram with no changes. I am completely baffled. Please somebody help. Thanks in advance.