。
I was really startled.
Literal
Truly startled did.
びっくり is a 擬態語 (state mimetic) evoking a sudden startle or surprise; paired with する it becomes 'to be surprised / to be startled.' The pattern [mimetic] + する is a productive way to verbalize many Japanese mimetics: どきどきする (be nervous), うっかりする (slip up), etc. Subject omitted.