I closed a shard by accident. Can I get it back?

Yes. crystl keeps a short undo-close list per gem, so a recently closed local shard comes back from the shard bar or crystl resurrect with its name, color, and best-effort isolation and agent session intact. (This is separate from the automatic session restore that brings everything back when you relaunch crystl.) See resurrecting shards. For recovering the agent conversation itself after a window or terminal closes, see where did my agent session go.