It's always helpful to provide your Roku model and model number when asking these questions. You can find them via Settings > System > About.
First thing is always to check the batteries in the remote to be sure they're not dead.
If you have a point-anywhere remote (off/on toggle button at top and volume up/down and mute on side), it must be paired to your Roku. If it's become unpaired somehow, see this video from Roku re how to pair a point-anywhere remote: https://www.youtube.com/watch?v=EK3T6eXqPU8
If you have a direct-line-of-sight infrared (IR) remote, be sure the front of your Roku is facing out toward your viewing area as the front is where the IR sensor is. Be sure you're pointing the remote at the Roku. Also make sure there is nothing in front of the Roku that could be blocking the IR signal. If you have a digital camera, even one built into a mobile device, you can check whether the remote is putting out an IR signal by pointing it at the camera and watching whether you see it flash on the camera view screen when you press buttons on the remote.
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