Since you do a lot of recording with your B2, it occurs to me that, since new recordings are not written to the spot where the operating software is stored, that the card could have become corrupt on an area that has been overwritten a number of times. If you are hoping to do quite a bit more recording, I suggest you buy one of the mircro SD cards that is rated for more wear and longevity, and flash the new card with new software. If this solves your current problem, then we'll know what the problem was. If not, then you'll have a more sturdy card that should last longer than might be expected otherwise.
Another possibility is that the recording you were making required more space than was available on the SD card, and it wouldn't be any great surprise if the B2 had not handled that situation gracefully, thereby corrupting the file that was being written. So to guard against that in future, buy a larger card - 64GB might be appropriate - of course, that will depend on the length of the recordings you do.
The instructions for flashing a new SD card are located near the top of the left column on the Techy Stuff page of the Brennan website, here:
Techy StuffThe file you download to be handed to the Etcher program must remain in its zipped format.
One last thought: For those who do a lot of recording, there are easier, more foolproof ways of doing it. Many swear by the Audacity program, which is a free download and has wonderful capabilities. I think it works on both PCs and Macs (DL the right version). Also with Audacity, you can edit the recordings in such ways as dividing album sides into separate tracks, noise reduction, volume amplification, etc. Then you can copy the files to a USB disk (formatted as FAT32) and Import the files onto your B2.