What happens if an American citizen tries to re enter the USA from?
US CBP will make every effort to determine if someone who represents themselves as a US citizen without documentation is actually a US citizen. After all, everyone can walk up and say I am an American. Let me in. After a thorough investigation the US citizen can enter the country. Thorough investigations take time and you will enjoy the hospitality of the US government in one of their holding facilities. A much better, smarter, more effective way is to go to the US Embassy or a US Consulate and present yourselves there. They help with any lost passport without being detained at the border. They have staff especially for that kind of situations and they are very good at their job. US CBPs mission is not getting you a new passport.