The zero button of my calculator didn't work

If this calculator is very old and dirty, the contact points on the board might need cleaning. This may be hard to do depending on the calculator, but you will want to unscrew the back of the cover, and use a jimmy tool to pry into the case. Move the jimmy all around the edges of if until it pops off. Take out the board. There will be a very large ribbon cable like thing with a bunch of wires running through it. You are going to want to take high concentration isopropyl alcohol (I'd recommend 90%) and clean the large ribbon cable on the side that touches the buttons. Reverse the disassembly steps to put it back together and you should be done with the repair.