Due to the nature of blockchain, transactions may be pending in the mempool (buffer for waiting transactions) for very long time (even for days).
Once transaction was accepted and sits in this buffer, you can only wait for the transaction to be either confirmed or dropped by the network. The more fee you pay, the faster will your transaction be processed by the miners.
You have to take into consideration, that block confirmation times vary by the crypto currency, for example:
BTC = 10 min
ETH = 10-20sec
Under extremely busy networks it can happen that processing times can jump up to half a day or even more! It is not unusual to see transactions pending for multiple days on ETH and BTC!