Hi, going to assume you're talking about bash_completion and not just auto-completing filenames, the more advanced bash_completion is made available by the package bash-completion, it sounds to me like that package just isn't installed, so first step is to check that. If it is installed and you still don't have it enabled then we're unfortunately going to have to suggest you check your bash resources; check your ~/.bash_profile or ~/.bashrc file if they contain:
if [ -f /etc/bash_completion ] && ! shopt -oq posix; then
That's the default one anyway, I mean you don't really need that whole if statement, you just need to source /etc/bash_completion really. Anyway, to get bash_completion going it's one of those two steps (or possibly both) that are giving you crap, and that's how you fix it.
If I have misunderstood you and your problem isn't really with bash_completion, but rather just basic tab-hitting to auto-complete filenames and hitting tab itself has stopped working, may I suggest that you might have befucked your ~/.inputrc - this file needs to be copied from /etc into your home and then modifications added afterwards, if you just add a single modifier, _all_ other bindings will stop working, the file needs to be complete.
If this still isn't the case I'd be rather interested in hearing more about your issue, sounds like an interesting puzzle (: