The Ethereum network can have large fluctuations in usage, causing swings in fee prices and resulting in some transactions to remain unconfirmed for long periods of time.
When this happens, please be patient and wait for the transaction to confirm. Sending new transactions will NOT make the current one confirm any faster. The way Ethereum was designed, new transactions are added to the end of the queue, so they will also remain unconfirmed waiting for the previous ones.
If you want a pending transaction to confirm faster you should follow the steps described here to replace it with one paying higher fees.