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.
Roku Community Streaming Expert I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well. If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X