All was going smoothly, with my 40D i've had it for years, i've used it with my 100-400 at three airshows this year last one being Shoreham. But i get to Dunsfold yesterday took around 6 shots and then error 99 strikes

Thank goodness i carry my 350D with me i was able to use my 100-400 on there fine.
Anyway i've tried all my other lenses on the 40D and still get error 99 i cant take any pictures with it. The camera is long out of warranty as i've had for a few yers now. Does anybody have any experience as to what it could be?
Is it expensive to repair as well????