General:
* Startle
* Jolt
* Shock
* Alarm
* Scare
* Frighten (although this is less specific about suddenness)
More specific:
* Jar (implies a sudden, jarring movement)
* Disconcert (implies a sudden, unsettling feeling)
* Unnerve (implies a sudden loss of composure)
* Appall (implies a sudden feeling of horror)
* Terrifie (implies a sudden feeling of intense fear)
The best word to use will depend on the specific context and the nuance you want to convey.