You didn't specify what model Roku you are using or your location. Depending on your model number, it's possible the channels you are trying to install are not supported on your model, or you might not be located in a country that the channel is supported.
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Have you tried anything to get them back? For example, unplugging the power and plugging back in might work. (I'm suggesting just a simple reboot, not a full factory reset, at least not as a starting point!)