Projects are saved to the account you were logged in with at the time. Look in the sidebar / project list on bolt.new (and at stackblitz.com if you had that account before). The chat URLs in your browser history are the best lead: open them while logged in with the same account.
If the first half was done logged out, that part may only have lived in the browser. Going forward: connect GitHub or download a ZIP at the end of each session.