The first two things I’d try (rather silly, but just to make sure): Make sure you’re editing an HTML file with the syntax set to HTML (bottom-right corner) and that entering koans (Alt+Ctrl+Enter) works. If this works then that means that you have ZenCoding installed and trying to use it in the correct situation, but something is interfering. Chances are, another package is gobbling the tab. (If entering koans does not work, disregard the rest of this post. Try removing and re-installing ZenCoding.)
I would then work in the opposite direction. Backing up my configuration, doing a revert (sublimetext.com/docs/2/revert.html) and installing nothing but Package Control and ZenCoding. If this works, then the problem is somewhere between a default installation with ZenCoding and your current setup.
Re-install your present configuration and use Package Control to disable packages (especially one that bind the tab key) tat you have recently installed to see if you can fix the problem. You can also find "tab"
in *.sublime-keymap
in your Packages folder to try and spot packages whose bindings may be interfering.
Some combination of the above should work – or at least exhaust what isn’t at fault
Hope this helps,
Alex