If you haven't quit the browser in awhile, this can happen. The good news is that in Chrome you can kill just the flash plugin without losing the rest of your browsing session (I usually do this by opening the Activity monitor and killing it from there; usually at that point you can see that the Flash plugin has gone completely nuts eating memory...)

Once the Flash plugin is dead, you can reload any open tab that needs Flash and it will come back better behaved. Until the next time it gets into whatever state causes that complete non-responsiveness.

Note that the alternative to this -- at least in Safari -- is just to have the browser crash completely occasionally. If you look at the crash dump, it's still the Flash plugin doing it, though.