For a normal AC discount code, I was be able to contact AC to have them re-issue the code again. You can try that and see if they will re-issue an AMEX Discount Code.
Good Luck
What was the rationale for reinstatement? I used an AC code on a latitude fare and when I wanted to change arrangements AC said the code could not be reinstated. I also used one on a J tx and when AC changed the schedule I needed to redo the tx. A very kind agent practically did cartwheels to reorganize the tx to keep the code. Lots of extra work for her. It would have been much simpler to cancel and rebook but she had no way to reinstate the code.
It would be really helpful to know for future reference.