General synonyms:
* Dumbfounded
* Astounded
* Amazed
* Bewildered
* Flabergasted
* Stunned
* Nonplussed
* Taken aback
* Shocked
* Dismayed
* Speechless with surprise
More specific synonyms:
* Overwhelmed: Expresses a feeling of being completely overpowered by emotion.
* Tongue-tied: Emphasizes the inability to speak due to nervousness or shyness.
* Gagged: Indicates a feeling of disgust or shock that makes speech impossible.
* Dumbstruck: Highlights the sense of awe or wonder that leaves someone speechless.
* Mesmerized: Suggests being captivated or enthralled to the point of silence.
The best synonym will depend on the specific context and the emotion you want to convey.