I dont have an answer to your question as I cant just open Maya and try and tackle it, I create a poly I extrude- it works!
I figured you might have accidentally hit something while creating it -Locking translate in X or something I dont really know much about, but I'm a noob! KNOW many here can help if you're still suffering. Like me they probably assumed you got on with it somehow. Sorry again.
If it was me I would have just deleted that piece of geometry and tried again to get on with the Tute, as it doesn't really matter. Please don't get me wrong, I have a prob at the moment that doesnt 'really matter', but I'm still bombarding Miss_Nova with questions because knowing what to do when something isnt going right will help next time (and theres always a next time) Love to help you if I can, - Have you read my Signature? I'd learn something.
So some questions:-
Can you create a new poly cube in that scene and extrude a face in the X?
Is snap to grid on? If it is, sometimes it's awful hard to move a handle from certain angles.
Deleting that 6 sided Poly and creating a new one shouldn't take long. I know it doesnt help you know what went wrong, but will get you moving again - does it still not work?? If not then I'm even more intrigued - try resetting the tool (that tool doesn't really have many- but you never know)
Are you willing to post your file?
Send me a PM if no-one else who knows what they're doing jumps in with the answer and I'll bombard you with more questions
Sadly I can't see an obvious solution which is why I didnt offer more help before.
IMO a new poly will work, and take 20 secs. The snap icons are at the top of your screen - They have little horseshoe magnets - they should not have squares around them.
If you select your poly and go to your Attribute editor, under 'Polysurface#' (# is a number) are any of your values greyed out? hit ctrl-a if you dont knowe where the Attribute editor is.
Umm yeah I'll shut up now...Hope something above works for you.
Learn from others mistakes, it takes too long to make them all yourself.