Matthew has a USB key, and while the computer sees it, he can't open any files on it. His backup is no better. Leo says that a USB thumb drive can get corrupted just like any drive and the good news is that he can recover it with software. It's probably just a corrupted file catalog and that can be fixed. He should right click on the USB key, select properties, then "scan and fix errors." He can also run Scandisk.
He may need to go deeper, and for that Leo advises Piraform's Recuva. The chatroom suggests copying the entire drive to his hard drive. Don't open it. Just right click on it and copy.
That's why Leo recommends having a 3-2-1 backup strategy: Three copies on two different formats with one off site. It's the best strategy for guarding important data.