This is how it is works internally:
1. The system selects the next order to convert into an invoice.
2. It "locks" the sales order so it cannot be released a 2nd time.
3. The invoice is presented to the user for confirmation. (optional)
4. If the user decides to continue, the invoice is saved to the database.
5. The system returns back to the original order and deletes it from the Back Order Control Centre.
If the software is interrupted between steps 3-5, the order will remain in the Back Order Control Centre. However, it will remain locked so that it cannot be invoiced twice, unless the user consciously unlocks it and invoices it a 2nd time.
Interruptions include: getting kicked off the network, pressing the 'x' button when the software was running (forcing it to shut down), or the software crashing for some other reason.
Assuming an invoice was produced but the order was not cleared from the Back Order Control Centre, then the failure must have occurred between steps 4-5.
The more usual scenario is failure between steps 3-4. If that occurs, the invoice is not
produced but the order remains locked.
In the more common scenario, the procedure would be to manually unlock the order and retry the invoicing process.
However, in your case, because the invoice has been created, the correct recovery procedure would be to delete the locked order, rather than unlock it.
Note the following which is very important:
If you must unlock an order establish why.
DO NOT IGNORE THE WARNING MESSAGE you are presented with. Establish if the transaction should be unlocked or deleted.
If an order is locked, is this because:
(1) Someone has the invoice open right now on another computer? (Locking is usually normal!)
(2) The session was closed on another computer without producing an invoice and the order must be released again?
(3) The invoice was produced, so the locked order should now be deleted, not unlocked.