There was a bug in 1.45 where tabs could disappear permanently under very specific circumstances, and you're the first person to report it in the field!  I actually happened upon the bug while looking at Simply Fortran's code.  It should be fixed in future versions.  Sorry for the confusion.

I thought about it and guessed that I might accidentally have dragged/resized the editor tab somewhere where I couldn't see it.

I ran regedit and deleted the Position Key from the registry and restarted Simply Fortran and it has fixed the problem.

Hope this is useful.

When you say you "tried reinstalling," are you referring to reinstalling Simply Fortran from scratch, or just adding the file to the project again?

What exactly occurs when you double-click you file in the project pane?  Does an empty tab open, or does nothing occur?  What happens if you navigate and select the file via "Open..." in the File menu?  If you navigate to the file using Windows Explorer, can you find it in that case?  Just trying to understand the issue.

I guess I might have dragged it somewhere that I can't see. Is there a way to get to the tab.