You can reactivate employees a couple of different ways - this depends on whether they have been archived (deleted) vs. made inactive.
If you are restoring an Archived employee, you can Restore the account on the user's Profile page, under the Admin section.
If you are reactivating an Inactive employee, you can do this under the Access section on a user's profile page by toggling the 'Active' setting.
Reactivating Inactive employees can also be done in bulk via an employee export/import. To do this, navigate to your Employees page, and toggle your employee list filter to All Employees. Then Options -> Export.
This will email you your full employee list where you can make any needed changes to the Active status column, save and re-import on your Employees page (Add -> Import).