On/off switch not working on iPhone - what is the problem?

Ok the problem is, sometimes when I press and hold the on/off button, for more than 10secs in these cases, the phone does not give me the slider to turn it off. Similarly, sometimes when it is off, the phone does not switch on when I press it.

Sometimes when I press and hold the button, I know there is a response because the phone comes out of standby. However it does not seem to "shutdown".

Question is, is this a hardware or software problem, and is there any way I can solve this?