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

  thunderstruck
      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 Oxford English Reference Dictionary (link)

thunderstruck
adj. amazed; overwhelmingly surprised or startled.

From Collins COBUILD Advanced Learner's English Dictionary (link)

  thunderstruck
    If you say that someone is thunderstruck, you mean that they are extremely surprised
    or shocked. (FORMAL)
   = stunned
   ADJ: usu v-link ADJ
  
  
  

From English Explanatory Dictionary (link)

  thunderstruck
  ˈθʌndəstrʌk adj. amazed; overwhelmingly surprised or startled.
  
  
  

From Webster's Revised Unabridged Dictionary (1913) (link)

  Thunderstrike \Thun"der*strike`\, v. t. [imp. Thunderstruck;
     p. p. Thunderstruck, -strucken; p. pr. & vb. n.
     Thunderstriking.]
     1. To strike, blast, or injure by, or as by, lightning. [R.]
        --Sir P. Sidney.
  
     2. To astonish, or strike dumb, as with something terrible;
        -- rarely used except in the past participle.
  
              drove before him, thunderstruck.      --Milton.

From Soule's Dictionary of English Synonyms (link)

  thunder-struck
  a.
   Astonished, amazed, astounded, dumfoundered.
  
  
  

From English Explanatory Dictionary (Synonyms) (link)

  thunderstruck
  ˈθʌndəstrʌk adj. dumbfounded or dumfounded, astonished, astounded, awestruck,
  awed, speechless, struck dumb, amazed, taken aback, staggered, stunned, shocked, dazed, numb,
  paralysed, aghast, open-mouthed, nonplussed, Colloq flabbergasted, floored, bowled over, Brit
  knocked for six: We were thunderstruck to learn that our team had been disqualified.
  
  
  

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

  58 Moby Thesaurus words for "thunderstruck":
     agape, aghast, agog, all agog, amazed, astonished, astounded,
     at gaze, awed, awestruck, beguiled, bewildered, bewitched,
     breathless, captivated, confounded, dazed, dismayed, dumbfounded,
     dumbstruck, enchanted, enraptured, enravished, enthralled,
     entranced, fascinated, flabbergasted, floored, gaping, gauping,
     gazing, hypnotized, in awe, in awe of, lost in wonder, marveling,
     mesmerized, nonplussed, numb, open-eyed, openmouthed, overwhelmed,
     popeyed, puzzled, rapt in wonder, shocked, speechless, spellbound,
     staggered, staring, stunned, stupefied, surprised, taken aback,
     under a charm, wide-eyed, wonder-struck, wondering