I deactivated a resource on a task and now need to reactivate the resource. How do I do that?

Jylene, 

This can be done at the Resource Panel on the Task. You will need to expand the Resource Panel and then click on the Gear Icon, Manage. Add the <Resource>.State field if not already added. 

You will see that the user's <Resource>.State is Inactive (Lock Icon). You can change to Active in that field to activate the Resource.