|"mother board and power supplies many times"|
Well. What didn't you change ? The monitor? The RAM ? The graphics card ?
How about you provide some make/model numbers of the components including the powersupply.
Did you try a different monitor?
Did you try to turn the monitor on without connecting it to the PC ? If it is working you should get a "Signal out of range" or similar on-screen message.
Goin' Fishin' (Some day)