Which video card did you put in, Unless it's really old win 8.1 should read it fine. Might have a bad card, When I do upgrades to any PC I only change on thing at a time so if a issue comes up like this I know what I just did and make finding the issue much easier. Get your PC back to the way you had it running, When running shut down and put in the new card and make sure you have current drivers and connect just the montor if that works then shut down and add the TV with monitor attached. If it fails through any step then we will know which step is causing the issue. Also delegated the driver, installed and do a fresh install after you put the card in, PC may be still using the onboard video.