I was making a voltage tester with the Uno and the input was A0. When I went above 10 volts into A0, that pin stopped working. Is it possible to fix the pin? If so, what do I do?
Your A0 pin is killed. Every I/O pin on a microcontroller is protected with a diode clamp, if the voltage on the IO pin goes above or below the supply voltage or very small time then it is clamped. But if it stays there for a longer time the clamp diode burns internally.