Do these steps....
1) remove your monitor cable and fix it to your motherboard VGA port...and restart your PC and see if it goes blank again...if you don't have motherboard VGA then follow next...
2) once you manage to login to your system as per your saying...click start menu and click the powerbutton while pressing and holding SHIFT key click the restart button...this will bootup on to a bluescreen 4th option says Boot in Safe Mode...do that and see if booting in safe mode makes your screen go blank...update after you try these 2 steps....