Normally, listings are deleted manually by the seller or by duplicate listings being merged.
If you deleted a listing in error, you may re-list it by going to Manage Inventory > Add a Product and searching for the UPC or the same ASIN the item was listed as before. Then, click "Sell this product."
If Amazon merged listings that should not have been merged, contact Seller Support to request that the listings be separated again. Provide the two ASINs that were merged, along with as much evidence as possible that the items are different (e.g. photos).