。
One of the airplane's engines stopped.
Literal
Airplane [possessive-の] engine [possessive-の] one [subject-が] stopped [regret-てしまった].
~てしまう adds a nuance of unwanted completion — an engine stopping mid-flight is clearly bad news. エンジン is a katakana loanword from English. 一つが marks 'one (of them)' as the subject.