Hmmm... I'm assuming you went thru the steps listed in the setup guide, I asked about Home Sharing because I think it some settings can become disabled when you upgrade iTunes.
I'm also assuming you've able to access the internet content (youtube, apple store stuff)
Anyway you should also double check the following:
- You've entered the same Apple ID
- You're successfully signed onto the same network as your iTunes computer.
- Are you connected via ethernet or wireless? If wireless, you might experiment by switching to ethernet temporarily.
All in all pretty straight foward - the only time I had trouble was when my iTunes had become horribly corrupted/confused.
If I were you the first thing I'd do would be go thru step by step in in ATV Setup Guide as if you've never read it / done it. : http://manuals.info.apple.com/en_US/Apple_TV_2nd_gen_Setup_Guide.pdf
Apples' ATV Home Sharing Troubleshooting page.
It will walk you through all things to check. It's available at:
There's also a thread here from folks who had trouble after one of the updates, back in Oct at:
That will bring up related Home Sharing issue links. Be careful to check the dates on all this, and read with a grain of salt - folks often offer solutions far more complex/risky than required.
Should all else fail, it's worth a call to Apple.