If display was okay before you cleaned/vacuumed the inside of tower then it is probably dust particles or cabling causing the prob.
things to try:
1. take out the cables and re-insert them firmly
2. a different monitor to see if it the monitor that has the fault or if it is the graphics card in tower
3. take out graphics card if you have one (as opposed to onboard) and clean any specs of dust from contacts (do not touch with fingers the gold pin contacts) and the slot it goes into.
4. make sure the graphics card is properly seated in its slot.
The usual way to clean is to use a clean paintbrush and holding a vacuum cleaner near (not touching any parts) to suck up the dust you have disturbed but everybody has their own favourite way.