It sounds like you've set a resolution and/or refresh rate the monitor cannot support so it is forced to drop into standby mode. Find out what refresh rates it supports (perhaps start with 60Hz for the moment - you can try out higher rates later with Windows switching back after 15 seconds) for the resolution you want and change the setting in safe mode (refresh rates are changed under Display Properties->Settings Tab->Advanced Button->Monitor Tab). If that doesn't work, try uninstalling the video drivers in safe mode, reinstalling after a normal Windows boot and then changing the display options before restarting.