Right click on My Computer and go to Manage. Scroll down to Storage then Disk Management. See if you can find your external drive listed on the right.. It might be Disk 1, 2, 3 etc and listed as blank or unformatted. Right click on it and see if you can assign or change the drive letter to an unused letter to make it visible.
Easeus also has a few other free data & partition recovery programs
you could try. Haven't used it, but I would try the partition recovery program first. The data recovery program is good, but extremely time consuming.