OK, have you tried restarting the computer? If not, I suggest you try that, because sometimes a system restart will set things right again. Because you cannot see anything on the screen, you will need to just power the system down rather that using the standard shutdown procedure.
Just press and hold the power switch for about 10 seconds. The computer will shut down. Give it about 10 - 15 seconds and then turn the computer on again.
If you still don't see the monitor, then I suggest you restart the computer in Safe Mode. Safe Mode is a special diagnostic mode of operation where only those essential drivers are installed at startup so that you can attempt to repair faulty systems.
To start the computer in Safe Mode, press the power switch to turn on the computer and then press and release the F8 key repeatedly. You should see a Windows start menu on the screen. Choose your regular login and Safe Mode. You may need to use the up/down arrows to select, and the Enter key to accept the selection.
If the screen is visible, the problem may be due to a failed monitor driver (though it should work in Safe Mode). If you're seeing the monitor, then I suggest you uninstall the monitor driver.
1. To uninstall the monitor driver, right click on the "My Computer" icon and choose "properties".
2. Click the Hardware tab, then the Device Manager button.
3. Click the + sign to the left of the monitor entry.
4. Right click on the entry and choose "uninstall".
5. OK the selection and restart the computer in Normal Mode. (Just use the normal shutdown/restart procedure).
Windows should automatically see the monitor as "New" and reinstall the proper driver for it.
If this doesn't solve the problem, please post again and we can try some additional steps.
Dwight