Does any other monitor work? if not than the video card may be bad. or the settings in the bios may be altered. first try this...unplug the tower, open the case and remove the bios/real time clock (RTC) button cell battery, press the power button to dissipate any voltage that is stored. Re-install the battery, plug tower in and power up. If anything was changed in bios everything would go back to default settings and you would have to enter set up to update date/time etc.