cmh2001:
I have a project where I feed a voltage supplying sensor to A0. The Uno converts it to a digital value and transmits it via serial cable to my PC and I graph the result on my PC. It works real well most of the time, but occasionally for some reason it just stops converting the input to digital and always delivers 0 to the PC.
Can't say whether it's a hardware or software problem at this stage. I suggest you post your code, and hopefully we can then rule out software as the cause.