Sounds like analog to digital converter in the MPU died
If is an older sanguino with a large black rectangular chip, you can just get a new chip eg [
www.emakershop.com]
I would advise getting from a reprap supplier, if you get it from a chip supplier it will not have a boot loader and you need hardware to put one on it.
If you have a 644p upgrade to a 1284p (its drop in replacement with more memory)