Well, not all dents are created equally. A small dent is likely cosmetic. However, the shock of a fall or a blow may have damaged the internal workings of the drive itself or dislodged an internal connection. Most external drives are just an enclosure surrounding a "normal" SATA internal drive.
If under warranty, I suggest you RMA (return) the drive. If not under warranty, I would take apart to ensure that both the power and data connectors are firmly connected.
Good luck!