Hmm... as you know your xbox controller should work straight away...
There have been a smattering of reports of players having intermittent problems with their game pad since the morrowind update. I heard that it was fixed by closing eso, plugging the controller into a different USB slot and then reopening eso.
If that doesnt work then consider downloading the free app 'x box accessories' from Microsoft app store. It allows some limited key mapping of xbox controllers, but it will also let you check that your pc is properly recognising your controller. If you can see your controller in xbox accessories then the problem is definitely with eso, if not it is a problem with your game pad.
Good luck, please let us know how you get it fixed.
#1 tip (Re)check your graphics settings periodically - especially resolution.