The 'Busy' message will flash usually when the camera is busy writing to the memory card, or buffering. You should try a different card as a first step. Ensure that you use a high quality card with adequate speed to match the camera. We should also try a reset procedure if that doesn't resolve the issue.
1. Turn the camera OFF and remove the card and battery.
2. Remove the lens and use the camera body cap to prevent dust entering.
3. Let sit for about 10/ 15 min.
4. Use a new, lint free micro fiber cloth to gently rub across the lens contacts.
5. After the elapsed time, install the items back in the camera and turn the power switch to <OFF> and <ON> again.
6. Using the Mode Dial, change from mode to mode, trying to shoot with the camera without an error message.
7. If you have one, you might also try a different Canon lens on the camera.
If completing the above steps fail to resolve the issue then we recommend that you send us the camera for evaluation and/ or service. The root cause should become apparent under factory service conditions. You can set up service for the EOS 40D on our web site, through this link.