I had a similar problem once after activating Ubercart modules. Certain pages would not render the theme, though all the content was there. I went into performance settings and optimized CSS and all was fine. I heard this was due to too many css files being called at once. The optimize CSS setting streamlines them.
Today though I went tweak a css file and the theme broke again. I restored my edits and still no luck. So I tried other themes including zen and garland and still they all seem to be broken; none of them are rendering though all my contend is there. I've also tried various settings under performance and cleared the cache.
I should also note I'm having the same issues in various web browsers.
I'm not sure it is relevant but I this is a pre-production site online. I moved the directory path recently, but other than a few absolute links I had to change all came out OK. But I wonder if the path to my themes somehow might be an issue?
Thanks in advance for any advice...