Well I sussed it but have no idea why it suddenly went like that. Basically if you were a CBT wobbler and part of the Naughty Corner group, you did not have permission to see any of the 'blocks' on the front page. So in a way the front page was displaying correctly i.e. with no content.
You might not have noticed but I should think that the upcoming events at the top of the forum list wasn't there either.
Anyway I went through each and every bloody one of the blocks, and there's a few, and added the correct permisions so you should be able to see all again.
I'd like to thank my invisible friend Dan Lewis for his help.