No ideas - What type of file is it??
I would have suggested that you restart your computer. Now when I say "restart" - I mean a complete shut down -
From the start menu choose "turn off computer"
Then choose "turn Off" NOT restart
After the computer completely shuts down - start the computer again.
This will give you computer a fresh "boot". This should also have terminated any running applications and should "free" files that were being used.
If that does not work, if you do backups - the other option might be to go to your last backup file and fall back to it - but only do this if your file was started before the backup. You will have lost any work that you did from the date of the backup - but it may be better than nothing. Good luck.