Don't know it off-hand, but check your manual. There will be error codes in the last few pages. Most errors I get can be fixed by removing the battery for a few seconds and replacing it, though I've only ever had 2 types of errors.
This is usually a lens compatibility error I believe?
My friend gets it on his 300D with some older lenses that he has... seems to be very unstable... he has a 14mm wide-angle lens by Sigma, but it used to work OK... now it gives him Err 99 most of the time... He had similar troubles with another Sigma lens...
error 99 is a catch-all code when the camera can't figure out what is the problem. One thing to try is to switch lenses. If it goes away, clean the contacts on the lens it happened with, and on the camera body. Try again. If the problem persists, it may be the lens. If the problem keeps happening regardless of the lens used or whether you've cleaned the contacts, it may be there's a problem with the body.
Usually just unscrewing the battery pack and screwing it back on or removing the battery/ batteries and then putting them back in will fix this... I find it happens more when I am in a humid environment.
I had this within a week of buying my ancient 300D - I was using a sigma lens not chipped for digital. But Canon took the camera and installed (?) Firmware. I only used digital lenses after that and the problem went away;)
