We have a contract to purchase a business the sent us a letter saying they are taking back the business since we were a week late on the payment.

I presume the seller financed the transaction either with a note or an installment agreement.  Missing one payment should not result in you losing the business and everything you've paid to date.  You can probably stop the seller from taking that action.  You should retain counsel asap so that person can review your contract, get the details of your situation and create a gameplan to help you keep the business.