I have a Roku express. I have always plugged it into an outlet with the included cable and adapter. I once got and overheating message even thought it was not placed somewhere smothered with other things. Now I keep getting a low power message as if I am plugged into the TV USB port, which I am not.
If you are still unable to resolve the issue, please send me a PM (private message) with your Roku account email address, and include the serial number of the Roku device from Settings>System>About along with a summary of the issue you are experiencing. I'll be able to assist you further from there.
Is there a light showing on your device? How are you powering the device? Is it plugged into the TV USB port, or is it plugged into the wall outlet with the provided cables? In addition, have you tried using another HDMI port on your TV as well as trying your device on another TV to see if you are seeing the same issue?
Please let us know and we'll continue assisting you from there!