horror-struck 
(also horror-stricken)
adjective
Briefly paralysed with horror or shock: horror-struck, she stared at him with frightened eyes...
- She stared, horror-struck, into his eyes and realised that his mind had turned.
- I backed up, horror-stricken, staring at the doorframe.
- England is considering the possibility that its World Cup dream could lie in tatters seven weeks before a ball is kicked, and a frenzied southern media is horror-struck.
Synonyms
horrified, terrified, petrified, frightened, afraid, fearful, scared, panic-stricken, terror-struck, scared/frightened to death, scared stiff, scared witless, scared out of one's wits;
shocked, shell-shocked, stunned, stupefied, awestruck, aghast, appalled
vulgar slang scared shitless, shit-scared