Why did my prepaid balance disappear and then appear again?

If you are using a prepaid card to pay for your parking transactions then every time you activate a parking session the system "freezes" the maximum amount you can be charged for that particular parking session. This is done to make sure that you cannot exceed your prepaid credit. If your starting balance is less than this maximum amount, during the parking session it will look as if your balance has run out. Once you end the parking session, and as long as you did not reach this maximum amount, the system will "unfreeze" the funds and refund the difference to your prepaid balance.