This happened because the network was full and you tx had to low fee, so that it got pruned from the blockchain mempool database after a while.
But as you can see there https://www.blocktrail.com/BTC/tx/911e63f1e674bc5ca50f39a3003e5a08307e04bfbdb3aa50f59fe3bc4f56a2b3 the previous tx which remained unconfirmed has been spent again from the same input but , this time, with an adequate fee to let miners process it in a reasonable time.
If you check first transaction link https://www.blocktrail.com/BTC/tx/172bbca8d4bad4401e9d732daf01c8c2e13319032cbb03573455e49d3a271e5e you can notice the label "Double Spend" in the "Confirmations" field and just below which is the TX where this input has been "double spended".
Pay attention this is a "pseudo double spend" cause effectively no coins are spent two times.