If the buttons move & can be pressed -
- Update the phones' software. LINK
- Perfom a soft reboot.
- Reset the phone. LINK
In the buttons are stuck -
Unless the button(s) has come off or you can see physical damage, you can request a Warranty Exchange if the phone is less than 24 months old (12 months for iPhones).
if out of warranty or physical damage is visible, obtain a quote for repair.