If you have either Windows Vista or Windows 7, it's a frequent occurrence with a lot of applications. I don't know if it occurs with Windows 8, but it wouldn't surprise me if it does. In any event, it's the OS, not the application. Usually, you just have to wait a bit and the message goes away.

Simple example: If you close the launcher after pressing "Play" by subsequently pressing "X", you will take the focus back away from the game. You will still see the Windows taskbar at the bottom of the screen as the game is launching. Usually, you'll also see a message -- referring to the launcher, not the game -- that ESO is not responding. A few seconds later and that message dialog goes away.

Guess it started responding again, huh?

It's the same sort of thing as what happens when you ALT-TAB out of the game.

Usually you see the message because MSWindows sucks in general. Open your Task Manager and browse the panes a bit; you'll see a crapload of processes and services running, all clamoring for CPU, memory, and bus access. Your application -- ESO -- is just one more process contending for resources. If you're using IE, Look for the process iexplore.exe; every freakin' tab you open uses a crapload of memory. Since ESO is a memory-pig, ALT-TAB just creates a situation where your PC has to wait for memory to be freed up by memory-swap to virtual space (hard drive).

Up the priority on the game when running it and you'll see the "Not Responding" message less often... or you'll get it for IE...

I wouldn't close the game when you see the message after ALT-TABbing unless you have to wait an interminable amount of time for the game to start responding again.

It's like having an argument with your wife... wait a few weeks and she'll cool off...