that you moved things around to clean I am moved to ask what you used for the cleaning and why you thought you needed to remove components rather than dusting and blowing the air around them.
If you got busy with a brush it is possible that too stiff bristles damaged the RAM slots. It is also possible that loose bristles or something else got into the slots and are preventing full seating of the modules.
It is also possible that the graphics card isn't fully seated and also possible that the monitor cord is loose or damaged.
What happens when you only use ONE RAM module? Check each of them one at a time to see if one has a problem. If none have problems try them in differend configurations that your motherboard manual shows are acceptable.
A few days ago I did some cable management and cleaning in my PC I had to take out the motherboard, RAM & graphics card. When I booted it up I had no initial problems, then I noticed games weren't running at smooth frame rates & they were lagging quite a bit. I did a system check, with "dxdiag" and it showed only 1gb of RAM although 3gbs are installed. I found a thread on a forum about this suggesting that the RAM cards were in opposite slots after the work I did. I swapped them over, but then got 'no signal detected' on the monitor: swapped them back again and still 'no signal detected'.
I've checked for loose cables & reseated the graphics car twice: still no joy.
I put this pc together myself. Specs below:
Asus M5A78l-M USB3
AMD athlon x3 3.4ghz
Radeon 6770 1gb GDDR5
500GB hard drive
Can anyone please help me with this?