OK - I've had this problem before, but can't remember how to fix it. Once I build and serve my site, I'm only left with a directory of files (with no index.html). I'm using jekyll 3.4.0 - is this OK, or should I consider updating? It's always helped me in the past, but I haven't posted anything on my blog for a few months now.
OK - I realised that the index.html file got deleted (the template version) - found it in a backup and it works great again.