Google's documentation says:
Pausing a line item suspends delivery but keeps the line item’s inventory reserved and keeps the line item in view. You can't pause archived line items.
Releasing inventory frees up inventory that has been reserved by a line item. This means our forecasting system knows it can use these impressions for other reserved line items. You can't release inventory for non-reserved line item types (network, bulk, price priority, or house).
So "Inventory Released" status is shown for paused line items for which the system is not even going to reserve impression.
This status can get set after pausing the ad, or when a line item is copied.
According to this thread at Google Product Forums, to fix it, you need to select Resume from More Actions on the line item.