。
Lightning struck a big tree in the field.
Literal
Field [gen-の] big-tree [on-に] lightning-strike [subj-が] existed.
落雷 ('lightning strike') is a Sino-Japanese compound noun: 落 ('fall') + 雷 ('thunder'). The construction [location]に[event]がある ('there was an X at Y') is the standard way to report events that happen to or at a place. Note how 落雷 is nominalized as an event with がある rather than verbed.