Bats will use chimneys as places to hibernate if the fireplace is not used. Try tapping on the fireplace doors around dusk to see if the bat stirs. The goal is not to disturb it enough to cause it to take flight, just to see if it makes any movement. It is likely just resting, but it is possible that it got tangled in the mesh. Try tapping first and then let me know how it went. If the bat is dead it will need to be removed, but we don't want a live bat flying around your basement, so for now don't open the fireplace doors.