Involuntary Celibacy is when a person find themselves unable to establish an intimate relationship, or to have sex, even though they want to. In fact, that's the main reason that it differs from other celibacies. (If that's even a word.) For example a priest may be voluntarily celibate for religious reasons. Incel (the short-form) is different from being single, and some researchers use six months as the standard to describe incel, while others say that the person must never have been in a relationship.
Incel can be caused by a person's lack of social skills and charisma, or by a low self-esteem which prevent an incel from approaching potential partners. Physical factors like ugliness, or a physical disability can prevent the desired suitors from approaching an incel, and contribute to their low self image. Mental health disabilities like schizophrenia, or anti-social personality disorder can factor into incel, as can social class factors like being homeless, etc.
The main problems with incel is that it can cause loneliness, depression, anxiety, and sexual frustration. These are often caused by societal norms in Western society that expect people in their twenties to have had some sort of sexual interaction; and the lack of which can result in mocking by peers, or feeling "left out". Another problem is a higher mortality rate than the general population.
Their condition is caused by the inability to not only have sex, but also not having the chance to kiss, cuddle, or be just be intimate in general. Incel can sprouts from missing key sexual experiences in adolescence and usually results in a high preoccupation with sexual activities and low amounts of social contact. (IRL, so e2 don't count!)
For further reading:
Involuntary Celibacy: A Life Course Analysis