If you have a Windows Phone 8, this is a trick you should know. No matter how stable the OS is, it still hangs. I am currently reviewing the Nokia Lumia 820 and it hangs once on me. I have no choice (at that moment) but to take out the battery to reset it.
However, if you have a WP8 that has built-in battery (cannot be taken out), here is a way to soft reset the phone. Press the volume down button and the power on/off button together for 10 seconds (see the picture above). It will automatically perform a soft reset. Hope that this information helps.