吓了一跳 (scared into a jump) is the most common collocation. 吓坏了 means scared stiff or scared to death.
grammar
Read xià when it's the everyday verb 'to scare' (the spoken, colloquial sense). Switch to hè in formal compounds about intimidation - 恐吓, 恫吓, 威吓 - and as the literary interjection of disapproval or astonishment.
Left mouth - the indexing radical. Anchors 吓 as something done with the voice: a sudden shout to startle someone. From that vocal jolt came the modern senses to scare and to threaten. Same vocal family as 叫, 喊, 吼.
Right 下 supplies the sound for the xià reading. The down-arrow shape also adds a faint semantic hook: a startled person ducks down or sinks. Same phonetic in 虾. For the hè reading (恐吓, 威吓), the sound link has drifted - the traditional form 嚇 took its phonetic from 赫 instead.
