Do you mean the LCD on the top of the body?
Have you removed all batteries, including the date battery? If not, please do that, cycle the power switch and shutter switch a few times and then let the camera sit for half an hour or so. After that, put the batteries back in and start over. If you still have the same problem, you need to get a replacement.
I'm curious what brought this issue on. Did you make any changes of lenses or other accessories? The reason I ask is that I'm on my second 40D, which started Err 99-ing and it is back for service now. I'm awaiting word on what the problem is. It would be really great if we could figure out if there is some combination of accessories that the 40Ds just don't like. I know that isn't a happy thought, but I am beginning to wonder.