Nearly panicked last night, thought my OctoPrint had suffered a major bug, and I didn't know what to do to fix it. I could load the page, see the webcam, everything. I could upload any .gcode file I wanted. But I could not load or load/print the file. The buttons were faded. The only solid black button was the Trash button. I don't know how many times I rebooted the RasPi, or how many times I restarted OctoPrint.

Then I hit on the brilliant idea of refreshing the web page. Success!

*headdesk*
I've said it before... I'm dense.