SoRCon 11 23-25 Feb 2018 Basildon UK http://www.sorcon.co.uk
The card for the lost location stays in your deck and is just a dead card.
If you have a card connected to the lost location, a means of transport, and enough military to start a siege, you can siege it back.
And if you win the siege, the card becomes useful again. In the meanwhile, as has been said, the card is completely useless. (Worse than useless, it occupies a space in your hand.)