On the Customer Portal, you can not see your Closed or recently Resolved tickets. You may have received an email with a link to the portal saying a ticket has been resolved, but you are unable to see it.
On the Tickets tab, use the drop-down menu (shown below) to select "All Tickets", this will show you all open, pending, resolved and closed tickets. By default, this is set to "Open or Pending" to allow a clearer view but to see older tickets you need to update it. Once change it will stay on this selection until it changed again.