In theory you don’t actually need to re-open a closed voucher as the system will let you process additional payments against a closed voucher if required.  You get a message to advise that the voucher is closed but it allows you to continue the process.

If you feel you need to re-open the voucher then as long as you are still in the accounting period that the voucher was closed in, you can re-open it. But if you have already moved forward with the accounting periods then you cannot reopen the voucher.

To re-open in the same accounting period, you need to go into the original invoice line for the service on the invoice and untick the Closed flag on the details page that opens. 

If the check box is greyed out then you have already moved into the next accounting period and the flag cannot be changed.