You can test each button on the remote by looking through a digital camera at the front of the remote when pressing a button. Each button press should cause a flickering light to be visible in the camera viewer. If there's no light when the button is pressed, then it's certainly the remote. Note that this is assuming it's an IR remote, and not a WiFi Direct.
See if there's anything stuck under one of the remote buttons. Usually if something is there you can see something around the button edges.
I don't know what model Roku you have, or how long you've had it. If it's under a year, the remote should be under warranty and Roku is usually really good at getting replacement parts shipped. If it's no longer under warranty, you have a couple of options. One is to order a new remote from Roku. https://www.roku.com/products/accessories/player The simple IR remote is $15. You can also find replacement Roku remotes at stores like Best Buy or other places that sell Roku players.
Your other option is buying an inexpensive universal IR remote. I was looking at what was available in a nearby store that sells Rokus as well as other electronics, and they had several IR remotes that listed Roku support for under $10. The only Roku players that won't work with a universal remote are the Roku Stick players, as they are all WiFi Direct only.
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