Used to express a deep level of distress or emotional pain, more than just being sad or upset. After hearing the tragic news, he was in bits.
Pro Tip 2/3
Informal Speech
It's a casual expression, so it's more suitable for personal conversations than formal situations. When I called her, she was in bits and needed a friend.
Pro Tip 3/3
Post-Event Reaction
Often used after specific events that cause emotional reactions, like a breakup or bad news. He was in bits after failing his driving test.