An unforeseen event is one that nobody expected to happen. The rain that ruined the outdoor wedding, the illness that emptied half the office, the delay that pushed the opening back a month.

In practice you meet it most in a small set of fixed phrases: unforeseen circumstances, unforeseen problems, unforeseen difficulties, unforeseen consequences, unforeseen delays, unforeseen costs. 'Due to unforeseen circumstances' is one of the most useful phrases in polite English — it is what a shop puts on the door when it closes suddenly, what an organiser says when an event is cancelled. It explains without explaining, which is exactly why people use it: it is a graceful way to say something went wrong without telling you what.

Almost always the event is a bad one. You can say 'unforeseen benefits' and be understood, but the word carries trouble with it, so unforeseen good news sounds slightly odd.

Grammatically, it nearly always sits in front of a noun. We say 'unforeseen problems'; we do not usually say 'the problems were unforeseen', though it is not wrong.

It is moderately formal — comfortable in a letter, a notice, or the news, a little heavy for chatting with friends, where you would say 'something came up'.