In the Web Browser go to Accounts > Invoices, then select the Show Archived check box at the top of the table. You can then search for the invoice you want to restore, and click on that check box to bring up the options at the bottom of the page. Select Unarchive Invoice(s), and the invoice will be restored.
Please Note: If you have integrated with Xero you will need to restore the invoice in Xero to ensure changes can be made in Formitize as the Invoice will appear as read only in Formitize until restored.