20D Error 99

Just turned the camera on and was focusing then got an error 99
So I took off the battery grip, took out the internal battery, took off my 18-55mm took off the flashgun then left it for a few minutes put the battery back in with one of the ones out of my grip and put my 70-200mm on instead and now it just won't turn on and the flashgun has stopped working also

any help??

Is the battery fully charged? Also, I would try with everything removed from the camera (grip, flash, lens, and compact flash), and see if any of those components are causing the problem.

When fault finding, it's usually a good idea to change one thing at a time... Not everything you've ever heard of being associated with the error all at the same time. One thing at a time narrows down the source of the problem. All at the same time doesn't.

Since you were focusing a lens at the time the error occurred, I'd suspect dirty lens contacts and try cleaning those first.

But now that it's more of complete shutdown...

Did you test the level of charge in the memory battery... or perhaps just buy a new one? These batteries might last 5 years, but also might need replacement before then.

How long did you actually leave both batteries out? I believe you should wait at least half an hour or so, to be sure to force a complete "reboot" of the camera.

What flash are you using? Some with high trigger voltage might damage the circuits of an electronic camera. (Note: I think the 20D is rated for up to 250V trigger voltage, and there are relatively few flashes that have higher voltage than that).

Did you replace the internal battery?

If it doesn't turn on at all and the batteries are charged you might check the CF slot door as well. If the switch is stuck the camera will not turn on.