When a supplier is inactive, you won't see them in the suppliers' list anymore. However, the supplier has not been deleted and can be reactivated through these steps:

        1. Go to Supplier Management → Manage suppliers;

        2. Click on Show filters, type the supplier's name, and choose Active - "No" Filter;

        3. Select the supplier you want to make active again click on the Edit button in the Action column; 

        4. Move the switcher on the right side of the 'Edit Supplier' row to activate the inactive supplier (a switcher should be green and show "ON")  press the Update button to save the changes.

Now the supplier is active and available.