Can I sue McDonnald's?

Yes. These types of claims are known as premises liability suits. Businesses have a duty to ensure the safety of their customers. This duty includes making sure that their property or premises is free from unseen hazards, like falling air duct grates. Before a complete evaluation of your case is made, we would need more information about your injuries and, particularly, medical treatment that you received or may need in the future. But based on what you have submitted here, you most likely have a viable claim.