Most of the "prepaid" phones - at least the US versions - do "expire" if not used for a while.

Typically here you have a "number of minutes" remaining, but also an "expiration date" (sometimes shown as "number of days remaining") both of which should be shown by the phone. If you drift past the expiration date any remaining time is lost. "Adding time" also "adds days" according to a formula that varies with the provider.

You should be able to purchase new time and re-activate the phone, using the same procedure used at the time of original setup. Some, but not all, providers permit you to request that the original phone number be restored, but the old number may already have been assigned to another phone if the phone has been expired for very long, so it's not always possible.

John