First thing, test it with another computer and another USB cable. If that does not work, you can try removing the drive from the enclosure and use it with another one or directly on a SATA cable in a desktop. If the issue is not with the drive, that will bypass the enclosure. In some cases the drive connection is custom or the enclosure encrypts the drive, in that case it's a lot harder to get data from it without having a prior backup done.