General synonyms:
* Dumbfounded
* Astounded
* Amazed
* Bewildered
* Nonplussed
* Stunned
* Shocked
* Disbelief
* Awe-struck
* Tongue-tied
* Mute
* Silent
* Wordless
More specific synonyms:
* Overwhelmed: If the feeling is one of being emotionally flooded.
* Floored: If the feeling is one of being completely surprised and knocked off balance.
* Speechless with awe: If the feeling is one of intense admiration or wonder.
* Dumbstruck with anger: If the feeling is one of extreme anger that prevents speaking.
* Speechless with grief: If the feeling is one of deep sadness that renders one unable to speak.
The best word to use will depend on the specific context.