A crisis is a difficult or dangerous time in which a solution is needed — and quickly. For example, the crisis caused by a natural disaster might inspire you and your friends to make a donation.
The noun crisis comes from the Latinized form of the Greek word krisis, meaning "turning point in a disease." At such a moment, the person with the disease could get better or worse: it's a critical moment. Think of a celebrity whose recent antics generate headlines like "Rock Star in Crisis" — that person needs help that may or may not be sought. At the moment of crisis, things are unstable and maybe even dangerous.