I have a Time Capsule (3rd Gen) and Airport Express 802.11n. The Capsule is hardwired to the router (of course) while the Express extends the network wirelessly from the Capsule.
When I tried to upgrade the Express to 7.6 from 7.5.2 via Airport Utility (AU) 5.5.3, everything looked like it worked excpt AU was never able to find any of the devices after the green light reapeared on the Express.
Rescanning came up with no devices.Relaunching AU didn't help. I even launched AU from an old iMac and it's AU couldn't see find anything either. However, both computers could get to the internet.
I decided to check this board for issues - looks like there are some and then not upgrade the Time Capsule just yet. Instead, I walked away from the computer and contemplated life, the universe and wireless devices.
Returning about 10 minutes later I was surprised to see that AU had found the network again. I then kicked off the upgrade to the Capsule and everything went perfectly.