if it's stuck on 1024 for a short time and then drops back to zero shortly therafter, then it's probably clipping: the piezo is putting out enough voltage to saturate the clipping diodes inside the Atmega8, and keeing the value read at 5V. That's one theory at least. Do you have a picture of the circuit? IS this a Zimbabwean thumb-piano kind of thing? We might be able to help more by seeing what this thing looks like.