Could be either a motherboard or a PSU issue, or maybe both are faulty.
There is no easy way to test the motherboard (without the use of special test equipment), so other components have to be tested to eliminate them as being faulty.
First confirm that all cables/wires are connected properly (this includes PSU auxiliary supply, and monitor cables).
And make sure the CPU, video card, and memory are all seated properly and evenly.
If still have issues then -
Test the memory in another computer, and/or try some other known compatible memory. Borrow equipment if necessary.
Test the video card in another computer, and/or try another known working video card. Again, borrow equipment if necessary.
Perhaps the easiest test for the PSU is to try it in another computer, and/or try another known working PSU. Borrow other equipment if necessary.