Click on the task that you want to reassign to the resource.
Then, expand the “Resources” panel, you’ll see a list of resources that are currently assigned or have been assigned and logged time in the past but are now “inactive”.
You can see this in the “State” column, if a resource is active you’ll see that green arrow, if inactive it’ll show the “on hold” sign.
Select the “state” cell on the inactive resource you want to reactivate, and change it to active.

Click on the task that you want to reassign to the resource.
Then, expand the “Resources” panel, you’ll see a list of resources that are currently assigned or have been assigned and logged time in the past but are now “inactive”.
You can see this in the “State” column, if a resource is active you’ll see that green arrow, if inactive it’ll show the “on hold” sign.
Select the “state” cell on the inactive resource you want to reactivate, and change it to active.

So simple - thank you Eric.