we had a few reports on corrupted cards, but we have never been able
to identify the reason.
Sometime restarting the browser could fix the problem, but I don't
think this is the case for you.
If you have an offline copy of your data it should be trivial to
restore it; otherwise get in touch with me that I will try to restore
a snapshot of the DB and let you try to access an old version of your
data, hoping your card is still fully readable.
Sorry for the trouble.
Best regards,
Giulio Cesare