Once a work order has been closed (by changing it's status to Closed), it can only be modified by users belonging to the Administrator, Service Manager, or Bookkeeper roles. However, users belonging to the Lead Technician, Technician, or Subcontractor roles can also edit closed work orders if granted the Modify when closed permission in:
Settings > Users & Permissions > Permissions.
A closed work order can be reopened by clicking the Reopen when viewing the work order. During the reopen process, you will be required to choose a new status for the work order, along with the option of adding a comment about why the work order is being reopened. Once reopened, modifications can be made to the work order as normal.
Customers can also reopen a work order via the Customer Portal. If a customer reopens a work order, the status will be set to an Open status of Reopened by Customer.