I had the same problem on TCL Roku TV. I have mine set to stay in standby when I hit power down. I had been "on" for 35+ days, which is about how long the thumbnails have been absent.
Do this power reset: Unplug your set and keep it unplugged for about ten seconds or more. Plug it back in and power it on. Navigate to "Prime" and you should have your thumbnails back.