Your specs would help, particularly what graphics card you have and, as this is a graphics issue that could potentially be related to a power supply problem, the model number and power rating of your PSU.
In the meantime (and I'm assuming that you are talking about a desktop here), try the following:
Check that the card is seated squarely in its socket, and that it is lined up correctly with the expansion slot in the case. The easiest way to do this is to remove and reinsert it (take the opportunity to blow any dust out of the slot). You might need to loosen the motherboard mounting screws slightly to achieve this (retighten them afterwards). Again, on the same principle, disconnect and reconnect any auxiliary power connector(s) on the graphics card. If the PSU is modular, do this at the PSU end of the cable(s) as well.
If it is a laptop, then it could be a cooling issue. Are the air vents clear, and are the fans kicking in when required?