Missing startup.txt error

I’ve run into a strange error that stops me from launching/testing my work in progress. When I open the index, I get an error which says ‘Couldn’t load scenes/startup.txt The file is probably missing or empty.’ startup.txt is still in my scenes folder and it definitely is not empty. I’m not sure what changes I might have made which would make it suddenly stop reading the file. Any ideas?

Edit: I moved everything from my external drive back to my C drive and that seems to have solved the problem.