If it's unreadable, it has already been demagnetized, that's what wiped out the data on it. Flimsy magnetic strip cards like that cannot be restored, you'll need to talk to the transit staff to see if their computer has your remaining balance and if it can be put on a new card.
You cannot demagnetize your metro-card if it is already unreadable. Once you pass it over a magnetic field, the data on it are erased and may be hard to be retrieved. Otherwise, you can consult a technician for more options.