If you have received an email notification/ticket update that your ticket has been marked as resolved (shown in the examples below).
Customer portal resolved ticket notification
Email notification of resolved ticket
You can easily reopen the ticket by replying to the ticket on the customer portal or responding to the email directly.
To reopen the ticket on the customer portal:
Add Reply
To reopen the ticket via email:
Send reply to the email in your email client (i.e Outlook)