Trout rarely if ever float when dead, the usually sink to the bottom. So if they did die, you wouldn't know it. Invest in a cheap thermometer, if your water temp is over 68 degrees, your rainbow trout will not be very active and very tough to catch. By about 72-74 degrees, they will be impossible to catch and much above that they'll be dead. This depends on DO levels as well, which are often the thing that kill the trout before actual temp does. I suspect your trout are still there, but not active enough to be easily caught. They're probably hugging the bottom if the cooler zone with just enough DO to hang on. Unless your pond is spring fed with substantial flow, I doubt it will hold trout year round.
