The Org sync will fail if the Salesforce refresh token has expired or been revoked, the Org was never authenticated or if the connected Sandbox Org was refreshed (and the underlying Org ID has changed as a result). You can quickly get the syncs running again by simply refreshing the token.
Do not disconnect the Org as it will delete the Org Model and all associated data.
Prerequisites for refreshing the connection
You need to make sure that you are already logged in to the appropriate Org before clicking the button. Otherwise, you will reconnect to the wrong Org.
Alternatively, make sure you are logged out of all Orgs, Trailhead, Appexchange, SuccessCommunity and SalesforceHelp sites. This is to ensure that you do not connect and grant a token to the wrong user ID (and possibly to the wrong Org).
Select the Org model from the list that failed to sync. You can either click on the hyperlinked action text in the list or select the Org settings tab in the right panel and choose the "Authenticate"* option.
*Depending on the context the action button may display "Authenticate", "Refresh or "Reconnect". In all cases, the result will be the same.
You will be taken to the Salesforce login screen. Log in with your credentials to the appropriate Org. After you click "Log in" you will be back in the Elements app and the connection will have been reauthenticated.