If you are using two(or more) different power outlets in your studio, and a piece of gear is powered by one and the mixer by another it is possible to create a feedback loop through the mixer. This usually happens if you maybe, lifted a ground (or some other voltage change). Anyways, if you did then the inputs could easily have been overpowered for a brief second.(melting soldering or arching from other grounded points on the board). I also think that you should open it up and check. It may be something simple that you can remedy yourself.(if your lucky)If not then I suggest sending it in. A power conditioner wouldn't protect the flow of voltage through the inputs. This actually happened to my tascam(Digital mixer) and I ended up frying a set of sub-buses. Then again the wiring in my home was older.