Simple:
* As scared as a rabbit in headlights
* As frightened as a bird in a cage
* As terrified as a mouse in a cat's paw
* As scared as a child lost in the woods
More evocative:
* As terrified as a ship caught in a storm
* As frightened as a moth drawn to a flame
* As terrified as a deer in the headlights
* As scared as a leaf in a hurricane
Figurative:
* As if their heart was a trapped bird, beating against their ribs
* Like a spider caught in a web, their fear paralyzed them
* Their terror was like a cold, creeping hand around their throat
The best simile for you will depend on the specific situation you're describing.