In my case, my iphone just freezed. I got a message and iphone showed its preview and then suddenly hanged. The clock was running, so as other processes. Tried to turn it off, but of no use. Pressed home button alot of times, nothing happened. It wasn't even going to standby mode.
So what is the solution?
We have to manually restart the iphone now. In my case pressing the power button for long time didn't work too. So to reset you just have to press both HOME button and POWER/WAKE/SLEEP button together for for some time. This way iphone will restart.