The cable was unplugged: Continue over Wi‑Fi

Second Display takes a pulled cable as your choice, so it doesn’t switch to Wi‑Fi on its own. The menu says “Cable unplugged”.

• Plug the cable back in: Second Display connects again by itself, as Thunderbolt or over the cable’s USB link.
• Click Continue over Wi‑Fi in the menu to go on over the network, with a compressed picture. It’s offered when the other Mac can be reached over Wi‑Fi or Ethernet.

If you reconnect within a few seconds, the display stays where it was and your windows stay on it. Later, Second Display puts the display back where you arranged it.