To reopen an invoice, follow these steps!
Locate that invoice in the ALL invoices tab and click into that invoice
Go ahead and edit that invoice (even if you delete a word and add it back in) - it should no longer be classified as CLOSED.
Review and be sure that everything looks correct - especially your payment schedule, the payments made to this invoice and balance due - just a quick once over.
You'll need to republish that invoice (even if you didn't make any real edits, and you always have the option to republish without resending any emails to clients) - which should set this invoice live and make it accessible again to anyone who has a link to this invoice (your clients). Republishing the invoice also enables any payment reminders that you have, so this is an important last step!