How old is the monitor?
Make and model of monitor?
Could it be that the new video card is incompatible with the monitor.
Make and model; of video card?
Have you tried the the monitor on another pc.
When a monitor fails, here in the UK, we can take it backstreet electronic (not pc) repair shop, where they check for free and give an estimate for repairing. Then one decides whether it is worth proceeding or not. Generally these shops are run by techies from abroad who are innovators.
Please advise back on progress.
Good Luck - Keep us posted.