You can use testdisk and Photorec from http://www.cgsecurity.org/testdisk-6.13-WIP.linux24.tar.bz2 to recover missing files from your SD Cards. There are other tools at http://www.cgsecurity.org too.
You can also install it from your distribution’s repository. It is a part of some digital forensics tools.
In Ubuntu 14.04 open up the Software Manager and look for photorec. When retrieving from a damaged USB device, files are recovered with arbitrary file names, though their extensions may be intact.