No Display on monitor

Hi I have a custom built cp that has been having some issues lately. one day I was on my cp and it crashed and didnt turn on so i bridged the main power cable so it would boot but when it turned on (fans spinning harddrives powering up) there was no display on the monitor. I then checked the gpu, ram, psu, harddrives and bluray player by swapping parts with my friends cp. All of those things seemed to be working fine.so i reasonably assumed it was my motherboard and not my cpu becuase of the rarity of a cpu failure. I had the motherboard replaced through msi's rma process. Yet when i installed the new board it didnt work, so I tried again. my pc was still not showing a display so i reassessed the problem and tried rma-ing the cpu. during that time I ordered a new motherboard from asus just to be safe. I installed the old msi board with the new cpu and it didnt work. it then took it in to shop and they had no luck. Out of blind hope i tried turning it on one more time and it worked! It was working fine for two weeks when my new motherboard arrived. I then thought that i might as well install the new asus board as it was of higher quality and proceeded to install the new board. I got the new board in and re-installed windows and was up and running perfectly for about another week. It was working flawlessly and I was very happy with the new board. It was working fine nothing out of the ordinary then i went upstairs to get a drink, came back downstairs and saw my monitor was asleep. I went to click and wiggle the mouse to get the monitor out of sleep mode but the screen remained black ... i turned it off and tried to reset it but no display was showing up on my monitor. I unplugged the dvi cord from both ends and securely plugged it in yet no display. I tried a hdmi cord instead of dvi yet no display. i opened it up and made sure ever thing was plugged in right yet no display. I am very angry and exhausted right now and need help. Why would it be working fine and the monitor just go to sleep and not turn back on. It doesnt make any f*cking sense!

please, if you have time i would appreciate input.