When SyncNow learns that its session has expired or was ended, it signs this PC out and shows the sign-in window. Sign in again with your email and the one-time code SyncNow sends you, and your projects come back from the server, where they are kept.
A session ends when you choose Logout on that PC, when you revoke it from another PC with Manage sessions in Account, when the account is deleted, or after a period without use, which is much longer if you checked Stay signed in when signing in. A session the server no longer knows also counts as ended, so SyncNow asks you to sign in instead of staying connected with a session that cannot work.
When this PC is signed out, SyncNow ends the sessions its agents opened from this PC and empties their transfer folder. After you sign in again, each agent opens its project again with open_project.