。
She fell head-first off the ladder.
Literal
She [topic-は] ladder [from-から] head-first fell.
まっさかさま combines the intensifier prefix 真っ with さかさま ('upside down, reverse'), giving 'completely upside down / head-first.' It functions adverbially with motion verbs like 落ちる ('fall'), 突っ込む ('plunge'), 倒れる ('topple'). The same 真っ that intensifies colours (真っ赤, 真っ白) extends here to a non-colour word — a productive pattern for emphatic adverbs and adjectives, with 真 ('true, genuine') and a geminate っ doing the intensifying work. 落ちる takes から for the source of the fall, just as 降りる takes から for the place one alights from.