Orthodontist did grinding and exposed dentin layer. Who do I see now?

Regular dentist. Return to your general dentist for evaluation and treatment.
General dentist. Ask your general dentist to place some composite bonding material in the exposed area.
Perhaps no one. If an orthodontist does an occlusal adjustment (grinding of the crowns of teeth) he or she did it to bring the force of biting into harmony. There can be many reasons why it could expose the dentin; if small area, keep it clean and do not fret. If hypersensitive, speak to them about remedies.
More dentin. The biting surfaces of teeth are comprised of enamel over dentin. If your occlusion or bite required adjusting, more than likely only the enamel of the tooth was adjusted unless you had lost a significant amount of enamel due to previous dental procedures, trauma, or parafunctional habits such as grinding or gritting your teeth. The dentin layer in the tooth is fairly thick.
Your general dentist. Go see your dentist. He/she will give you your options.