We "park” (deactivate) credits after a long period of account inactivity. This helps keep your credits safe in the event your account is compromised: When this happens, you’ll receive an email notification and your account statement will also be updated:

To unpark (reactivate) your credits, submit a support ticket including your BLEND profile ID and we’ll help you as soon as possible.  

Important: after 24 months of profile inactivity, available credits on the account will be canceled and cannot be restored.