Siri not working iPhone

It could be the server is either down or under going maintenance but try resetting the iPhone by pressing and holding the sleep/wake button and the home button for 20 seconds (maybe longer) until the Apple logo appears, ignore the red slider.  Then turn back on. You won't lose any data doing the reset.

I seem to be having the exact same issue....two iphones, one works with siri and siri on the other iphone has not been working for 4 days now. That is one long server issue if that is the case.