If its a continuous thing and nothing is really working out for ya, then you probably have to much going on in your computer such as memory wise. If you try to minimize alot of what your doing, it can help. getting rid of old unnecessary things will open up more RAM memory for ya.
If that's not the case, then your video rendering card is probably not working to well. if possibly try to reinstall the game again and see what happens. Turn down your levels to the very last, and go from Open>Direct. Turn off all water reflections and limit the view distance. It might be limited, but at least you might be able to get in.