Did you accidentally Vacuum him? Or under Sand? I have no idea of a Pleco's behavior. Never had one before. Need to learn more about them, I guess. If he IS hiding somewhere, you could lure him with some Veggies, right?
Do they even eat Vegetables? *I Feel Stupid* If they do, find Veggies that they REALLY like. Maybe Cucumber? O.o
Over hundreds of bettas die in their small, dirty cups each day. Only you can help them live. Research and then save them, today!