A faulty monitor would also cause the same result.
Or it could be the CPU's are not sutable for the board, assuming that the two CPU's are the same, tried on two identical boards.
Or perhaps the Power supply.
If you give more detailed info about the cpu's, boards, monitor, other hardware, and what you have tried to do to fix your problem, someone may be able to help.
At the moment all that is knows is that you have a problem, and that is is no signal to the monitor.