Made a very long WIP gamebook offline, it was working fine. Saved it. Now it's saying "Invalid XML: Invalid character in the given encoding. Line 1, position 1." Oh, and Notepad++ can't read the code for some reason.

Is a .aslx file encoded obscurely by default? I can't view it in the desktop editor, before anybody suggests that.

It worked fine before this. Desperately need it to work; I spent many months on this gamebook.

It's possible if the computer crashes while the editor is saving that it will open the file to write to, and then not finish. This could result in the file being truncated.

The most common reason for this particular error is trying to open a blank file.
This could be because the file is unreadable for some reason (faulty disk?), or it could be that the 'save' routine replaced the file with an empty one ready to write out its current contents, but then crashed or was closed before going any further.

What kind of error does Notepad++ give you? Trying to track down that error could help diagnose filesystem or permission problems. If it just shows you an empty file, then you're probably out of luck – this means that the file may have been erased by the computer shutting down at the wrong moment, or the Quest editor being forced to close unexpectedly.

It may be possible to get the file back using some kind of undelete or file recovery tools; I've done this before when I had a similar issue with Excel. But I've not used Windows in nearly 20 years, so I suspect the tools available now are quite different from the ones I remember.

It may also be possible to recover something from temporary files, but again I'm not sure how you'd go about that. I suspect that Quest might create a temporary .quest file when you run a game for testing; and if you can find that, it could be possible to extract the ASLX file from it. But it won't be easy.

Notepad doesn't give an error, just an unreadable file.

The code seems to be unsendable through this forum, which doesn't sound good.

There is a lot of text. Which is good because my file had a lot of text.

That sounds very strange. Are you able to share the file and post a dropbox link or something? I'm not sure what the best way to approach this would be.