DEF's freezing point is about 12 degrees F and if you find yourself in those temperatures it's not uncommon for DEF to freeze overnight. There is generally nothing to worry about, as your vehicle should start up and run normally. Most DEF tanks are heated and will melt the DEF so you can drive without issue. However it is suggested that one doesn't completely fill the DEF tank during the winter months, to better allow for expansion as it freezes, even though most all tanks should be built for expansion.
DEF will not be harmed by freezing, nor will their be any dilution issues as it unfreezes. Whatever you do, DO NOT ADD ANYTHING TO YOUR DEF TANK, EXCEPT PURE DEF.
Winter weather is approaching, enjoy.