|You have two separate machines and likely two separate problems.|
Since you have not given us the specifications of either machine we are guessing but I suspect that your original PC has both an onboard graphics port and a plug-in card and when you re-connected your monitor you accidentally connected to the onboard port. You do not get a signal error message because the port is electrically in circuit but no signal is being sent there. The graphics signal defaults to the plug-in graphics card.
Now the unknown PC that developed the graphics problem after you started it up.
Does it have onboard graphics or a plug-in card? If the former, I suspect the motherboard has failed. If the latter then I suspect the card has failed.
You need to separate your two problems and work on resolving them one at a time.
Please provide a detailed description of each system.
Goin' Fishin' (Some day)