when turning computer on screen says no signal. Worked fine yesterday

Things to try:

a) A new VGA cable (or DVI cable, depending on which is appropriate).
b) A different monitor.

if neither a) or b) fix the problem, reseat the graphics card (unless it's an integrated one).

And if nothing above works, consider that the graphics card may have failed. It's not a driver issue because that would not cause a "no signal" error.