Depends on the conditions of the ride. If it's super muddy, clean it. Buy a can of brake cleaner from an auto parts store, spray chain and cassette. Relube. Ride. It's just a bike. Common sense tells us moving metal parts need to be reasonably free from large particles and should have some form of lubricant. Your cables could have stretched a little as well. See if turning your barrel adjustors helps tighten up the rear shift.