These things are just special purpose computers. Like their big brothers sometimes clearing things out by restarting them can correct problems. (Note that if yours is a Roku TV with "Fast start" enabled, turning it off and back on just puts it into and out of a standby state and does NOT give you a complete restart.)
Depending on your Roku model and Roku version number you'll find the System restart option under Settings > System > Power or if not there, under Settings > System
If a restart doesn't do it, or if you don't have sufficient control with the remote to navigate the Roku menus, you can also restart by pulling the power plug to the Roku for a few seconds then plugging it back in and waiting for it to restart.
