I recently attempted at building a PC, and everything was moving smoothly until I moved my RAM from my old PC into the new one. After I finished building the PC I booted it up and all the fans were going, computer was on. But there was no display on the monitor.
Would a broken RAM effect video?
More aboutwould broken effect video
What RAM do you have? Maybe it's not compatible.
Computer worked fine for a week with the same specs on an old build.
Pull out your RAM, and put them in a different order in the slots, or pull them out see if nothing is bent/chipped and clean out the slots? Maybe something got in there? A few pieces of cat hair on my rig stopped it from running.