This happens to me occasionally when I have changed the colour/transparency of a vismat material and/or I have an imported model with an unrecognised texture.
In the first case, I tend to save what I am working on and exit, when I restart it all worls fine.
In the latter case, if you are using an imported model, it may be worth removing the textures from this model and re-applying them with either native or vismats.
Hope this helps