If a serious crisis were to occur, the government would have to act quickly.
Literal
By-chance serious crisis [subject-が] arise-[conditional-たら], government [topic-は] quickly act must [probably-だろう].
生じる ('to arise, to occur') is a formal verb, more literary than 起こる. It's typically used in written or official contexts for abstract phenomena — crises, problems, opportunities. The obligation ~なければならない combines with だろう to hedge both the requirement and the future outcome.