From WordNet (r) 2.1 (2005) (link)

  dumbstruck
      adj 1: as if struck dumb with astonishment and surprise; "a
             circle of policement stood dumbfounded by her denial of
             having seen the accident"; "the flabbergasted aldermen
             were speechless"; "was thunderstruck by the news of his
             promotion" [syn: dumbfounded, dumfounded,
             flabbergasted, stupefied, thunderstruck,
             dumbstruck, dumbstricken]

From Merriam-Webster's Collegiate Dictionary, 11th Edition (2003) (link)

  dumbstruck adjective
   Date: 1765 made silent by astonishment
  
  

From Oxford English Reference Dictionary (link)

dumbstruck
adj. greatly shocked or surprised and so lost for words.

From English Explanatory Dictionary (link)

  dumbstruck
  adj. greatly shocked or surprised and so lost for words.
  
  
  

From Moby Thesaurus II by Grady Ward, 1.0 (link)

  63 Moby Thesaurus words for "dumbstruck":
     agape, aghast, agog, all agog, amazed, anaudic, aphasic, aphonic,
     astonished, astounded, at gaze, awed, awestruck, beguiled,
     bewildered, bewitched, breathless, captivated, confounded, dumb,
     dumbfounded, dumbstricken, enchanted, enraptured, enravished,
     enthralled, entranced, fascinated, flabbergasted, gaping, gauping,
     gazing, hypnotized, in awe, in awe of, inarticulate,
     lost in wonder, marveling, mesmerized, mum, mute, open-eyed,
     openmouthed, overwhelmed, popeyed, puzzled, rapt in wonder,
     speechless, spellbound, staggered, staring, stricken dumb,
     stupefied, surprised, thunderstruck, tongue-tied, tongueless,
     under a charm, voiceless, wide-eyed, wonder-struck, wondering,
     wordless