I don't think it's usually possible to reach the spund panel from the pause mode, but can you reach the clock screen?
If you can, it's probably the A button, but if you can't, it might be a screen freeze, or the B button, or a combination of A and B buttons.
My tip is, unscrew all the nine screws carefully, and take out the buttons to rub the underlaying parts with a little bit of alcohol cleaning. It happens sometimes with tamagotchis, when they are exposed to heat (eg. on a sunny day) that the underside of the buttons melt a tiny tiny bit when pressed down on the circuit board. This usually doesn't brake the buttons themselves, but creates a black, sticky substance on the area where the buttons are pressed down, which will dry and then cause the buttons to malfunction. This was the case with two keitai tamas I bought recently; they were sold as broken, but this was their only fault, which was easily solved by a little bit of alcohol cleaning.
Otherwise, it might just be some regular dirt under the buttons, or in case it's just a frozen screen you should try resetting and downloading first.