Transactions can take some time to complete, depending on how busy the network is and what fee you have selected when confirming the transaction. You can choose between: Slow, Medium, Fast and Ape (reeeeal fast). The pending transaction will still appear under your 'Transactions' tab and you would be able to do a few things with it.
If you have a pending transaction, you have 3 options:
1. Wait it out - it will confirm eventually. You wouldn't need to do anything in this case.
2. Speed it up - simply select a higher fee and you'd be able to overwrite the transaction and replace it with one with a higher fee
3. Cancel the transaction: you'd be overwriting it with an 'empty' transaction. Please note you'd still need to pay transaction fee for it!