Yes - go to PO-D, highlight the Closed PO on the opening list and click the Reopen PO button.  You may have to go to PO Defaults, click Receiver Defaults, and set the default to allow closed POs to be reopened.
 
This is assuming you are on version 2002.1 or later.

You can try re-entering it in PO-A but if partials have been received, you may have to use Maintain Database to create the BKAPPO record with the correct PO number, then you can use PO-A to fill in the details.

Go to the right hand side of the table for the last record in the database and hit Tab.  This will start a new record.  Be very certain that all fields are formatted the same as the existing records.  When you are done all the fields and hit Tab at the end, the record will be saved.