There are five different versions of "Roku 2". Four of them are no longer supported. Even the one that is still supported is 10 years old. We would need the model numbers of your players to determine which they are.
Frankly, you've gotten your moneys worth from them, and they should be replaced. More than likely you've had something within the hardware board finally fail. Any cost to repair would far exceed the cost of a replacement Roku, and your current players can't touch the performance of even the basic Express (model 3960) available now.