Blizzard Balance has never been permitted for Subscriptions. You may be thinking of Game Time, which is a separate product.
Available Game Time options were recently changed. You can read more about that here: Changes to Purchasable Game Time Options
You can reactivate from the character select screen assuming the characters on your realm have enough gold in their inventory (not mail) to cover the current price of a WOW Token. You do not need to log into a character to do this.