If I am understanding you correctly, it was set on dash (or floor) and reverted to defrost on it's own. Pretty sure yours is still vacuum operated and defrost is the default setting. I'd start by looking for a vacuum leak. Remember, the parking brake release is on the same vacuum circuit, so be sure it releases when put in gear. I not, it's probably a ruptured diaphragm. If that's not the case, check all the under dash vacuum lines as well as the line on the firewall to be sure you have vacuum to the cabin.