Do you have Xfinity as your internet provider? If so, your problem may have been caused by a recent update to many Xfinity-supplied routers:
Xfinity users report the router configuration needs to be switched back to b/g/n security. If you don't have access to your router settings, you may have to request Xfinity to do this for you.
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