。
Unexpected difficulties came up.
Literal
Unforeseen-[classical-ぬ] difficulties [subject-が] occur-[past-た].
A short plain-form statement using the literary 予期せぬ ('unforeseen, unexpected'), where ~せぬ is the classical negative form of する with attributive ~ぬ. 起こる ('to happen, occur, arise') is the intransitive counterpart to 起こす ('to cause, raise up'). Useful minimal pair: 困難 ('difficulty, hardship') is a Sino-Japanese noun, while the related verb 困る ('to be troubled') is native Japanese.