Sounds like you abused the power rails in a way that trashed the 16u2 - this is a common failure mode on official boards and clones using the 16u2 (the clones using other serial chips are significantly harder to damage). Unfortunately, replacing this part at home is not practical unless you have hot air rework equipment and the experience to use it - and usually if the 16u2 goes, at least one other part is taken with it (such as the opamp for the power switching circuit)