If it's completely losing power it could be the power supply (the wall adapter on the power cord) that has gone bad. I have had this happen once with a very old Roku years ago and it sometimes happens with other devices too. You can order a new power cord by going to Roku.com and look up by your model number or do a search on Amazon with your model to buy a new one (even generic ones work well and are not too expensive.)
If your Roku is connected to USB power on the TV but you also have the wall adapter and never used it then try the adapter.
Is your Roku device powered by plugging it into a USB port on your television? The ports on many televisions only supply 0.5 amp, which is not enough to power a Roku reliably.
It is recommended that you power your Roku device via the power adapter that came with it, plugged into your house power. If your cord plugs into a USB connector but you no longer have the adapter that came with your device, you may have another one handy that can be used -- just make sure the tiny print on it confirms it has a 5 volt dc output of at least 1 amp.
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