So if the system has been reassembled, a picture should definitely appear. By mistake, most people sometimes do it the wrong way round and connect the HDMI cable to the connector on the board, HDMI or display port. If a CPU was purchased without VGA, no image is output there. So you should make sure that the board is reset once, and then with only one latch of RAM and possibly, if available, a graphics card that has been running before to ensure that it should start normally. What could be more likely is that a pin was damaged when installing the CPU. The most common error, however, is if you try to start the old system, which cannot or could not use UEFI, by switching off the UEFI mode. Then you also get no picture. If it still does not work, just write to us and we will find a solution.

Which ram kit are you using?
I had such a ghetto with the ram, but afterwards everything worked, even the picture came, after the ram problem :)