When a work order has been closed, and you want it to to be reopened, you 'reissue' it.

This functionality is available through the Admin1 license. It is not available through the WOx technician (such as WO4 and WO7) licenses.

This also means that, if you are an administrator, when you are in technician mode, you lose this ability.

But you can easily get it back by changing back into admin mode

First, navigate to the closed work order.

You will see it says the page is read-only. But if you have the rights, the page is only 'mostly' read-only. As my favorite movie (Princess Bride) says "[He's] only mostly dead"

Go to the Action and press reissue.

Then go and assign it. You can either do new assignments, or you can modify, most importantly clearing the 'completed' date from the existing assignments.