If you're using the TV's USB port for power, try the supplied power adapter. If you're already doing that, try another one from a phone/tablet that can supply at least 1 Amp. If you have another TV, give that a try. After that, about the only thing left is a factory reset. If it's less that a year old, there's always a warranty claim.
How to factory reset your Roku® streaming device | Official Roku Support
Step-by-step instructions for returns and warranty replacements | Official Roku Support
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