Your bacteria probably died while it was sitting.
It could also be the bacteria death coupled with something like Blue green algae - Cyanobacteria which thrives in stagnant low oxygen environments.
Cyanobacteria is in my opinion one of the worst smelling things in the aquarium hobby, only second to the smell of dead fish.
My tank had what I believe to be the same smell when I had a few fish die without me being able to find them for about a week.
The filter is made out of plastic so bad stuff could be stuck in the small pores of the material so I would scrub it several times and let it dry out between cleanings.