I just made this mistake myself and was scrambling trying to figure out how to recover the recording! A few things I learned along the way that I want to pass along in case they are useful to someone else.

First, here is the method that worked for me. I am using a 32R mixer and I recovered on a Windows 10 computer.
Insert USB drive into computer
Find drive in explorer and right click on it and select properties.
Go to the tools tab
Use error checking to scan the drive

Close properties and go into the drive in explorer
At this point, you probably only see the 0kb wav file.
Type * into the search box to show all hidden files on the drive.

Hopefully see a file with a name like FILE0000.CHK
Copy this file (or similar) to your hard drive
Open VLC Media player and Import this file into Audacity as a raw file with the following settings:
Signed 24 bit PCM
Byte Order: Little-endian
Channels: 1 Channel (Mono)
Start Offset: 0 bytes
Sample Rate: 96000 Hz

You should now have playable audio that you can edit and save as you wish.

In case this method doesn't work for you, here are a few things I learned along the way:
1 - A useful tool to create a backup of the SD card is called Win32 Disk Imager. You can download it from here:
https://sourceforge.net/projects/win32diskimager/

2 - It was not clear to me that the backup file of the SD card was supposed to be imported as raw data into VLC. This partially worked for me but the results were nowhere near usable for my purposes. It is possible that this method will work too, but I did not bother to test it once I found the solution that worked for me.

3 - When importing the raw file, the audio was slowed down. This is because I used the wrong sample rate. Slow audio means you need to increase the sample rate. Doubling or cutting it in half is a good starting point.

I think that's about it. Good luck to anyone else stuck in this situation!