Existential crisis, derived from Existentialism, is the psychologic panic and discomfort experienced when a human confronts questions of existence. This phenomenon, presumably, is common to technologically-advanced cultures, wherein physical survival is not life's priority.
An existential crisis results from:
Existential crisis resembles anomie (a sociologic concept), the mid-life crisis is an example. Usually, an existential crisis stems from the person's accurate perception of existence, i.e. Life is what it is.
In a non-existential belief system — religion (usually) — the essence of what it means to be a human being is mostly believed to have been defined before birth, usually by a supernatural being or supernatural beings. Typically, a lack of religious faith is prerequisite for suffering an existential crisis: the sudden awareness of not knowing the meaning and purpose of one's life, and consequent awareness of impending, inevitable, personal doom.
Cognitive dissonance occurs when the man or woman faces the paradox of believing his or her life important, whilst perceiving that human existence is meaningless and sans purpose. The person's resolving said paradox results in the existential crisis. For some people, a resolution to this crisis is the aggregation of religious beliefs into their personal existence. Analogously, Existentialism posits that a person can and does define the meaning and purpose of his or her life, hence must choose to resolve the crisis of existence.
Existential crisis is provoked by a significant event in the person's life — the death of a loved one; a life-threatening experience; drug abuse; adult children leaving home; reaching a personally-significant age (turning 30, turning 40, etc.); imprisonment in solitary confinement, et cetera. Usually, it provokes the sufferer's introspection about personal mortality, thus revealing the psychologic repression of said awareness.
The solution of an existential crisis requires the sufferer giving a meaning to life, e.g deciding that conscious thought is pointless in achieving existential truth and psychologic security; that it is unimportant to be aware of the world about one-self and how life works. To the sufferer, all that matters is the specious present, thus, being happy is pursuing a superficial life, whilst others might strive to learn more, to increase their knowledge of the world, et cetera.