If you cannot take the Roku device to another TV and access the Display Settings to change it back, or use another HDMI port on the original TV to get a signal again, try performing a Factory Reset. (Press and hold the Reset button for 30+ seconds).
If you are currently using the TV USB power port for power, try using a 5volt, 1amp USB power adapter at the wall (any cell phone 5volt,1amp charging adapter) prior to the factory reset just to rule out a power issue as well.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660x, Premiere+ 3921, Express 4k+ 3941