Someone who's incommunicado can't communicate with other people, either because they don't want to or because they're not able to. If you spend the weekend on an island with no cell service or Internet, you'll be incommunicado for a couple of days.
When someone is arrested, they're briefly incommunicado, although legally they will eventually be able to make at least one phone call. Your parents will worry about you if you're suddenly incommunicado, especially if they're used to hearing from you several times a day and reading your regular social media posts. Incommunicado was coined in the U.S., from the Spanish incomunicado, "deprived of communication."