"Failed to open" file whilst it worked fine yesterday

[Hudson555x]

Your file is not recoverable.
You’ll have to use a backup copy if you have one.
This is what your file looks like in Notepad ++

When it should like this:

You can read more on it here:

"Failed to open" error message after updating

This file is corrupt and it has nothing to do with the upgrade. The only way to recover is if you have a backup. You can read more about it by searching here on “NUL” or the web in general on “windows file null character” to find something like this. As far as we can tell this is not due to a bug in Shotcut but something in the way Windows does caching or hardware. Shotcut first writes a temporary file, verifies it valid XML, and then renames it to the name you provided.

In the future I would advise you to do File > Save As after you initially save the project. This allows you to save the file to different folder/file locations along with changing the file name.