If you are in fact able to at least get to the login screen, you can change your rendering system by clicking Options>Video>Rendering System. Then restart your game. See if that helps?
Have you downloaded anything recently? Maps, presets, objects, items, etc.?
Have you tried both rendering systems? You may have clicked the one your system was already starting up on.
If the problem persists, uninstall FH entirely. This means every single file associated with the game. You may have to search for some to ensure they're all totally gone.