I think it is either the batteries running low or the circuit board being damaged.
But if it resets and you see the egg, just select download.Change the batteries first,and if it dosen't fix it, well,unscrew the back with the 4 screws and check if a wire is damaged or if the circuit board is damaged.That's all I can say.
Sorry if I am not too precise, I have never had this problem and I am not an expert in taking apart and fixing tamagotchis.Well, I hope I gave you an idea on how to fix it.