Does it feel like it's somewhat stuck when you press it? If it does, give it a good cleaning by following the advice from [post|458355|this answer|new_window=true] by Reed Crosby; you may also want to follow this [link|https://it.ifixit.com/Guida/Sanyo+VPC+S1275+Function+Set+Replacement/2328?lang=en|replacement guide|new_window=true] and open the camera (steps 1-5) to check if there's anything under the button preventing it from working correctly.
The button may as well be physically broken: to confirm this, first ensure the control panel buttons are in good condition, then complete said replacement guide and reassemble the device, apart from the function set strip. Now turn the camera on and test the OK btn, you can navigate through the menus by pressing directly on the control panel using a pointed tool like a pencil. If everything works, you just need to buy a spare part and replace it (or salvage for parts from a broken device); otherwise there's a more serious damage and the device needs to be inspected thoroughly
Hope this helps, feel free to ask if you have any more questions