1. Motherboard mounting.

Why? Folk usually pull the old board and slide in the new one without checking if there are posts that short out the new board.

2. Any jumpers on the motherboard. As the make and model are omitted I can't check if it has jumpers that must be set or a slip of paper/plastic on the CMOS battery that is removed to energize the CMOS clock and more.

3. A forgotten power connection. Too many rebuilds show up with the ATX12V or other power connection forgotten.

4. The mic's LED is not an good indication since there are 3 to 4 voltages that must come up to turn the PC on. The mic LED is one rail.

5. I've also found folk think that because the CPU fits, it is compatible. Sorry but no.