External Hard Drive become small and all files missing.

Hello,
I have a problem with my 500GB external drive. I tried to recover my Windows 8 on my Acer Aspire V5 using Acer recovery Management. Before recovering it to factory setting, I backed up Applications to my external hard drive using 'create Drivers and Applications backup' and 'create factory default backup' on Acer recovery management. During factory default backup, I cancelled the backup and then started recovery to factory setting. After recovery I see that my external drive's capacity become 31,9GB free of 32GB capacity, and all my files in it were gone. Can anyone help me recover my data and disk space, please? Thanks.