If you cannot see the screen still, press CTRL+ALT+F2 to get you to a log in prompt. Type in your username and password. Then go to /etc/X11/
There is a chance that a video driver might have been updated and changed the xorg.conf file. Check in your /etc/X11/ folder, and see if there are any backup files to xorg.conf. It might be something named xorg.conf.backup or something similar. Check the file and usually towards the bottom it will show what your monitor resolutions are.
If the backup file looks like it has information pointing toward your monitor(s), copy over replacing the xorg.conf file.
sudo cp xorg.conf.backup xorg.conf
Then reboot your system.
sudo shutdown -r now