@Jaem
Make sure the HDMI cable is firmly attached to both the device any the TV HDMI port. (try unseating cable at both ends and reattaching). Try using another HDMI port or cycling/toggling through the Inputs using the TV remote (not the Roku remote) to verify that you habe the correct HDMI input selected.
If still a no-go, try replacing the HDMI cable if you have another available and also try connecting the Roku power cable into a cell-phone charging adapter and plug it into a wall outlet (if you are currently using your TV USB for power).
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