Don't write anything else to the drive. You can try some free data recovery software like the free version of Recuva
, but many people have better luck with R-Studio
. With R-Studio you can download and try it to see if it is working for you before purchase.
Since it is an external drive, it could be a corrupted drive due to a disconnection during a drive write, happens all the time with USB drives -- so never trust them as a sole source of backup for important data. It could also be a problem with the drive enclosure, or with the drive itself.