First of all check out "Is the Camera Button Stuck in the depressed position?" but if the button is not depressed and if it looks proper then its the internal hardware problem.below the button there is a circuit point the button is got moved from its exact circuit point or the circuit point itself is got faulty so it needs to checked first by disassembling the unit.if the button is moved from its position then it needs to be reseted manually or the other option if circuit point is faulty then that part where circuit is got short that needs to be re soldered.this will solve the issue. the unit is mostly fitted by screws so you can easily open the unit by unscrewing the screws by screwdriver.
This will help. Thanks please keep updated.please do rate the solution positively .thank you for using fixya

Hi there,

Well provide some more information, like - how old is your camera? Did the camera have proper battery power?
anyway the problem is most likely in the button itself, you need to open your camera and then try to clean the button (the button chip inside the camera), you can open this using a screw driver (there are 6 screws to open the camera).
please post comments for more help or information......

Good Luck!!

Thanks