So shocked that you cannot react — no words, no movement, just a blank moment. It is what happens in the first seconds after news you never expected, good or bad: you won; he died; she said yes. That is worth saying twice, because learners often assume it is only for bad news. A 'stunned silence' after an announcement, a 'stunned' winner on stage — both are normal. The word is strong but completely ordinary; people say it every day. Notice the shape it takes: 'I was stunned', 'stunned by the news', 'left everyone stunned', and as a describing word before a noun — a stunned expression, a stunned crowd. There is also the literal use it grew from: to stun an animal is to hit it hard enough that it cannot move, and a stun gun does this with electricity. Understanding that helps — the emotional meaning is that same helpless frozen state, produced by news instead of a blow. In British and Australian speech you may also hear 'stunning' used to mean beautiful, but that is a separate branch and does not mean shocked.