For milder reactions:
* Startled
* Surprised
* Alarmed
* Nervous
* Anxious
* Uncomfortable
* Disconcerted
* Flustered
* Apprehensive
* Worried
* Confused
* Shocked
For more intense reactions:
* Terrified
* Panicked
* Horrified
* Distressed
* Agitated
* Frightened
* Scared
* Overwhelmed
* Paralyzed
* Frozen
* Shaken
More specific alternatives:
* Bemused (puzzled or amused)
* Dismayed (disappointed or discouraged)
* Intimidated (frightened by someone or something powerful)
* Flabbergasted (completely surprised)
* Dumbfounded (speechless with surprise)
* Taken aback (surprised and confused)
Figurative language:
* My blood ran cold.
* My heart skipped a beat.
* My stomach dropped.
* I felt my knees go weak.
Ultimately, the best word to use will depend on the specific situation and the emotions you want to express.