Hard drive lost files

I recently installed Mountain Lion on an external drive I was using as a file storage, but now, having put that drive into my computer the OS is no longer seeing the files I had backed up on the drive, I'm presuming because they're not in the file directory.
Is there any way I can access these files in situ, or will I have to boot from another drive to retrieve them?
Thanks for any help.

Thanks for the response and sorry for the slow reply.
Yes, the files were still accessible, and there is a corresponding amount of space showing up as being 'used' on disk information about the drive, but no files being visible via the OS finder.