First thing I would check is to make sure that the computer or DAW is still using your interface as the sound card. For example, the DAW could be routing the audio to the computer's internal audio card, which could be turned down.
I don't know Logic, but somewhere there should be a menu item for selecting the hardware it uses to play the audio.
Sometimes an interface will have a switch for sample rate and if the wrong sample rate is selected, it will be silenced.