After automatic update of mnemosyne app (on my Samsung A40) it showed 0 cards.
Bad surprise for me. I suspected some fault with my phone. I didn't notice update of this app.
Unfortunately I do not sync my app with PC every week.
For others who will run into such situation, this is how you can recover:
1. backup PC data (just in case)
2. transfer default.db + journal file from phone to PC (directory data for you Mnemosyne)
3. perform sync from Phone: start app + fetch remote data
After deleting "Mnemosyne" folder on my phone and sync I was quite surprised that I cannot find new folder nor default.db.
.... and I found this thread.