yes, first of all you you did not clarify how it connected to mainboard, through PCI-e slot or through riser board?
cause it might dead riser..
but first what you have to check - it the gpu sits tight and all of cables are good and tight. ( so remove the card and insert it back gently)
then check connections to the monitor.   - if it is not help - then the best way to send it to service I'm afraid.