The error you're seeing indicates that the v0 preview sandbox session has been terminated and is no longer accessible. This happens when preview environments reach their session timeout or are stopped.

Here's what you can do:

1. Restart the preview: Come back to v0 and click the Version Box (preview button) in the chat to start a fresh preview session. You can then generate a new shareable link.
2. For persistent sharing: If you need the app to stay accessible long-term, you should deploy to Vercel using the "Publish" button in the top right of the v0 interface. This will create a permanent deployment that anyone can access anytime.
3. Why this happens: v0 preview links are temporary and meant for testing during development. They have limited session lifetimes and will eventually expire if not actively running.

Would you like me to help you with anything else on the app, or do you need guidance on deploying it to Vercel for a permanent link?