A transaction is sometimes dropped from the Ethereum network for several reasons:
- If the transaction was in the pending state for a very long duration
- It has a very low gas price as compared to other pending transactions
- The maximum number of pending transactions the Ethereum blockchain can handle are in the pending transaction pool
Once a transaction is dropped, all the gas and ETH is returned to the originating wallet:

Transaction dropped on etherscan.io
The dropped transaction is not included in any block on the blockchain, as if it didn't happen or wasn't initiated. There is no trace of those transactions on the blockchain; ...