This might not fix it but it did for me and don't jump off the roof in astonishment at yourself if it works -- you're not alone. You have to turn Home Sharing on. But it is on, you say. Well, maybe.
For the concise edition, just skip down and read the part in all caps. Otherwise, read on.
Problem: You only occassionally use Home Sharing on your Apple TV (ATV) but when you use Home Sharing it works a trick until one day you go to use it and it doesn't work. Variables (things you did between the last time you used Home Sharing and it worked and now when it doesn't work): Upgraded your Mac or PC, reinstalling iTunes or restoring your system from a Time Machine backup (Mac) or other type of backup (Mac or PC); otherwise restored some or all of your system from backups; updated iTunes via Software Update or by manually downloading the new version and installing it; did something else you have no idea what it was and I don't know either. UI inconsistency: iTunes 11 (and maybe 10, as well) Home Sharing preferences don't clearly and obviously reflect that Home Sharing is really TWO services -- a local network service you turn on in iTunes 11 in preferences in the Sharing section and an iTunes Store service authorized by your Apple ID that you must enable in iTunes 11 under the File menu at the Home Sharing item.
If you're like me, you have Home Sharing turned on in iTunes preferences. Prefs are set to share the whole library, no password, and status shows "on, no users connected." So it's on, right? Sort of. The local iTunes network service is on but you still have to enable Home Sharing via the iTunes 11 File menu item (in previous version of iTunes that supported Home Sharing I think Home Sharing authorization was instead located in the Advanced settings in iTunes preferences or under the Store menu, possibly both places in different older versions of iTunes).
I bought a 15-inch Retina MB Pro and restored my files from a Time Machine backup. All my iTunes prefs came over, though I'm pretty sure I had to log back into the iTunes Store to authorize the new computer to my Apple ID. If you just restore to your old computer and it's already authorized you probably SHOULD have to do that but you might not have do that, further complicating your impression of your iTunes Store Home Sharing status. Whatever the case, I was signed into the iTunes Store but I never reenabled the iTunes Home Sharing service in the File menu.
MAKE SURE YOU HAVE ENABLED HOME SHARING FROM THE FILE MENU IN ITUNES 11. TURNING IT ON IN PREFERENCES IS NOT ENOUGH EVEN THOUGH PREFERENCES REPORTS IT'S ON. "On" in prefs means your local library is shared from iTunes, it doesn't mean the iTunes Store authorized service is enabled for that iTunes library on that computer. No, it's no very clear, but that's the way it is.
A special note for MacBook users and any Mac users who regularly sleep their Macs on an all-Apple wifi network. If you want your ATV -- or an iPhone or iPad or another Mac or PC, for that matter -- to be able to access your shared iTunes library even when your Mac is asleep, remember to turn on "Wake For Wi-Fi Network Access" on the Energy Saver panel in System Preferences. Any Home Sharing device trying to hit your library will wake up your Mac so it can serve the iTunes library. (This works in a wifi environment that fully supports Bonjour. That's all recent Apple wifi gear and some 3rd party gear but I've no idea what 3rd party gear in particular nor do I know how to configure Bonjour-compatible 3rd party gear. In an Apple-only wifi environment it works by default.)
This feature is very handy and wonderfully Apple-ish convenient but it freaks out some MacBook users. The lid's closed! It'll fry! Yeah, well, no it won't unless there's some other serious hardware failure going on too. My 15-inch Retina MB Pro stays stone cold all the time, although some older MacBooks do get pretty hot sometimes for no apparent reason. But, for starters, Apple has long time supported using MacBooks in "clamshell mode" while driving an external display. That means fully powered AND driving some often whopping external displays, which takes a lot of graphics oomph. For merely Home Sharing your iTunes library, it doesn't even turn the screen on -- not even if the lid is open in sleep mode -- and I doubt it powers the graphics card. My external USB backup HDD spins up but spins right back down; it's only responding to the USB ports powering on. Serving up an iTunes library over wifi off HDD or flash storage is a pretty low-power deal. Remember, it's not PLAYING music or video on your MacBook; it's just streaming the files from your MacBook. Sleeping, lid-closed MacBooks in enterprise environments constantly get hit in the middle of the night over wifi for system maintenance and updates. Those can take 2 minutes or 2 hours and they're often a lot more processor-intensive than streaming an iTunes library.
Oh and remember you can stream photos to your ATV the same way, but that's managed via iTunes, too, likewise in the File menu -- not in iPhoto where you may be looking for it.