Transactions are kept in a wallet file along with the private keys for addresses. Your wallet.dat simply stored it for a very long time. As I assume you want the coins and want that transaction to go through, you could rebroadcast it and if the fee isn't high enough, use the CPFP method to get it confirmed.
You should go to Help->Debug Window->Console and type
gettransaction ""
, that should give you the hex that you can push to any online service like this one
https://blockexplorer.com/tx/send.
So I tried blockexplorer and it returned an error: "Missing inputs. Code:-25
blockchain.info gives me "missing parents for while inserting:
Could it be that this was spent from another unconfirmed transaction and since that one didn't confirm this one wouldn't show up? Or for some other reason was pruned by the network?
Any chance you can post the address here? Altough it probably won't be in a block explorer if it has been unconfirmed for thislong as they're normally cleared fairly quickly.
I think that the transaction you got probably got a double spend rom it also as that wasn't a bad time for transaction confimations even without a fee (as far as others say anyway).