Be lost for words
Definition 1 of 1
Pro Tip 1/3Emotional Intensity
Used when emotions are so strong that one can't articulate thoughts, not just confusion. 
Seeing the Grand Canyon for the first time, I was lost for words.
Pro Tip 2/3Unexpected Situations
Often used when something happens that you didn't predict, leaving you speechless. 
When the magician made my watch disappear, I was lost for words.
Pro Tip 3/3Positive or Negative
Can express both good and bad shock, such as extreme joy or dismay. 
The surprise party left me lost for words.