My transaction has been pending for a long time

Once a transaction is submitted it is in the network's hands, not the wallet's. Most settle in seconds; a busy network, or a fee chosen at the Standard preset, can take longer. The pending entry stays pinned at the top of History until it settles.

Open it and use View on block explorer — that is the authoritative answer about what happened, and it is the same information support would look at.

Do not send the same transfer again while the first is pending: you may end up having sent it twice. And be very careful with anyone who offers to “unstick” it for you — a stuck transaction is a favourite opening for a seed-phrase scam.