KrisKros.....that happened to me.  I put a check in the box  "Disable Post Lighting"  and "Disable Shadows" in the Graphics settings. Everything came back.You may have to experiment by placing a check in several boxes if that does not work for you.  I have better frame rates now.
It is beta and will be allot better when they get done.
Make sure you have the latest drivers for your graphics card. 
