Any changes or updates to do with vid card?
Can you enter BIOS?
Try and older CRT monitor if possible, if you can get a safe mode boot to OS, you can try a system restore.
Or uninstall the vid card driver
in Device Manager. Try using windows default vid driver to see if that works.
Consider the vid card driver as well as the Syncmaster driver, if one available, when updating.
If you can get a boot to OS,
start>run>msconfig>boot tab>check safe boot and network. reboot
Maybe, somehow an incompatible refresh rate has happened. If a CRT is an option, and you get in, goto display and make sure refresh at defaults.