For warranty information, try:
Step by step instructions for returns and warranty replacements | Official Roku Support
However, the first thing to check is the power adapter. If you're using the TV's USB port and not the supplied power adapter try the supplied power adapter. If you are already, try one from a phone or tablet or even give the TV's USB port a try.
Also, Roku devices don't turn off. That power button on the remote is strictly for the TV. Some devices go into a "power saving" mode from which they can be awakened by pressing Home on your Roku remote a couple of times.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution."
If you appreciate my answer, maybe give me a Kudo.
I am not a Roku employee.