When you switch on your computer, immediately start tapping the F8 key - keep on tapping it until the computer (hopefully!) boots into the Windows Advanced Options Menu (white lettering on a black screen).
On this menu, one of the choices should be 'Last Known Good Configuration'. Using the up/down arrow keys, highlight this option, then hit Enter.
If you're lucky, Windows will find the settings that last worked and start up.
The above may or may not work - it depends what the fault is. But it's the first thing to try.