I upgraded the Mollom module on this site from the 7.1.x version to the newer branch of 7.2.x and ran into a White Screen of Death (WSOD). Checking the logs pointed me to the Mollom error:
Class 'MollomDrupal' not found
It's tough to fix this as you might not be able to even log into the site to get to the admin page. Therefore, you will need to disable the module from the database.
Once you are able to log in, you should clear all caches from