Likely, it may be lost forever since it'd probably overwrite the file itself, but if you haven't saved much blockland saves or download large files after the incident, you may have a chance. Download a program like Recuva and try to find the save file and if your lucky the save will be in decent condition. The faster you do this, the better chance you may have to recover the file in some form.
The most important part of this; if your main hard drive is a SSD, the file is unrecoverable. If you have a HDD, and if your very very lucky, the save will still exist in some form in the hard drive.