Have you tried booting into safe mode, sounds like you are getting the login screen, try holding Shift down while clicking on Restart.
Once in safe mode you can try to remove video drivers (or just go to device manager and uninstall the display adapter), then booting as normal.
Both AMD and Nivida have options to reset all driver settings when installing a a display driver, I think you should opt for that once you get back into normal mode.
If all else fails.. use the Shift + Restart method to launch System Restore and restore to a point before the updates that caused the issue.