IT FIXED ITSELF! , nevermind and thanks. 
Things I have tried
The files exist on my hard drive (phew) and I’ve tried choosing different folders/files to use as the opening file, but I’m not having any luck. Help!
Ok, good to hear. Be careful and keep backups. 
If you happen to be using iCloud, it is known to sometimes move files off your local HD. Hopefully this was just a weird theme glitch or something.
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