If a customer sales order (COA) has been cancelled, it is not possible to undo this.
There are several jobs that run in the background when cancelling an order, and in some cases there can be for example an invoice plan linked to an item that would need to be handled.

In case a customer sales order (COA) has been cancelled by mistake, the user will have to create a new customer order (COA) and if an invoice plan has been involved, credit this and create a new one.

There is an option to 'Copy sales order to new sales order' in the context menu on a sales order in status 9.