The tape was cut.
I feel lonely as soon as the phone line disconnects.
This knife cuts well.
This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb.
This is an intransitive verb. That means it does not take a direct object. The kanji means cut so this means something is cut. Something breaks or snaps. No direct object here. That's why this verb means to be cut.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you to remember the き portion:
Something is snapping. Turns out that something is your arm. Your arm is a 木 (き) that grew out of you one day. "What's this ki doing here?" you ask. Then it suddenly breaks in half! "OW OW OW! My ki! It's been cut!!"