Flac file error

Good Afternoon I hope you can help?

I've used MP3tag for a while without issues. I was recently trying to add album art to a FLAC file. For an unknown reason it was taking an unusually long time and not completing the task so I decided to abort. When you now try and access this file through MP3tag, all the metadata has disappeared. When I try and add the metadata, an error occurs, it says the file "cannot be opened for writing".
When I look at the file in Explorer, there has been a .tmp file created. Neither the FLAC or tmp file can be deleted as it says they are open with another program. There is full read and write permissions on the flac file,

So I am now stuck with a FLAC music file with no metadata that I cant delete and cant edit?

For the time being I would take that error message serious.
You would have to investigate which other program accesses the file and hinders MP3tag to write back the file.

Have you tried closing all programs, restarting the computer, and then trying again?

Something is creating this issue, and unless you find/fix it, the most certain way to resolve the "other program" is to restart the computer. If that does not work, then you probably have some kind of corruption of the file or the drive it is on.