Please post your code and tell us which chip you're using. Without that it's next to impossible to give you any help - except for what Bruce already have given you. It doesn't matter if it worked two days ago and now not, post your code so we can take a look at it.

If this is a large program, try reducing it as much as possible to illustrate the problem.

Set ANSEL/ADCON, CMCON, TRIS correct?