There's definitely a power button, but they're getting harder and harder to find. Search around the area of the manufacturer's name/logo. On my TCL it's a good inch behind the "TCL" logo in the middle bottom of the TV. Assuming your TV isn't made by Roku (few are), you should contact the manufacturer (onn, TCL, etc.).
Where can I get support for my Roku TV™? | Official Roku Support
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