Hang on my 7D
I had something similar happen to me with my 2.8 100mm.. I attached it.. and nothing seem to be working and made my camera hang.
I removed the lens and check its contacts.. they were dirty.. I also checked my battery..
And until now i have not had a problem.
So check your contacts on the 50mm lens, and clean them. Check the battery contacts too and just wipe them with a cotton buds/qtips.
If the camera is still giving issues try a different CF card.. also you could maybe run the fw upgrade again.. and follow the instructions to the letter..
if still nothing.. you better call Canon.