Well, the main thing that is probably confusing you is that git keeps track of complate versions of the entire repository, not just one file. So what you have to do is figure out which version of the repository has the version of the file you are looking for, and roll back to that.
If you just want that old version of the file, but want all the other changes that the repository took in the meantime, you have to do a bit more work.
For complex operations like rollbacks, I'd generally suggest using one of the GUI tools (like git gui), as they let you visualize better what you are getting ready to do.