Check all the connections to your motherboard. Make sure you have your 24-pin motherboard connection fully seated, the 8-pin on your GPU fully seated, and the 4-or-8-pin connection for your CPU fully seated. The GPU and CPU pins often have these little tabs where you join two connectors together (4+4 to make an 8-pin) and if those aren't oriented correctly you won't have a good seat.

What is your PSU? There's usually an amperage draw of around 25amps on the +12v rail for the 3060. If you have a cheaper PSU, that could be a potential issue. You can look at the side of the box/side of the PSU to get what the +12v rails maximum current is (for a 750w it should be around 60amps). If that number is far lower, that could be your issue, but I doubt it unless the PSU actually has a problem.

If you want to take it to basics, remove your RAM from the PC. It should error on boot. There should be an indicator on the motherboard using lights (near the 24-pin connector). If you aren't getting any POST codes with the RAM out, your motherboard may be the problem. Plug the RAM in one stick at a time and boot the PC to see if it fails RAM on any of the slots. If everything is plugged in and passing, make sure the GPU light on the POST isn't flagging any problems.

You can take your motherboard out of the case, and assemble on the box it came in. The box won't short the system. If it works, then you may have a short when you secured it in the case.

Beyond this, you may need more specialized help by providing exact details of what you're seeing/experiencing.