What happens if the appraisal comes back lower than the agreed upon purchase price?

For example, your offer to purchase a house is $300,000.   The appraisal comes back with a value of $280,000.    The Lender will consider the value of the property to be $280,000.     You can contribute more of your own money to make up the difference or you can try to go back and renegotiate with the Seller to purchase the house for the $280,000 appraisal value price.  You can begin the process again with a new Lender hoping the appraisal will come back higher.   Or you can walk away from the deal.