From the path of the error message, I'm wondering if you have had a failed upgrade.
I've been updating my Concrete5 sites manually, so my updates folders are normally empty.

As a test, perhaps temporarily rename your updates folder to something else (like updates_temp) and see if that clears your problem. If not, you can just rename it back and look at other possibilities.

I'm not 100% sure of how the updates folder is used, so others may be able to give you a better answer. The actual error message itself points to a database object not being loaded in, perhaps suggesting that the database connection is dropping for some reason.