maybe late, but hope it will help somebody.
had the same problem.
I think you just pressed the button too hard and it broke down.
but it is easy to repare
you should disassemble the cover button from it's inner side.
do not cut the button rubber from the outer side.
there is a very simple mechanical mechanism with flat spring inside the button, and if you press the button too hard, usually it happens accidently, the spring deforms and do not function properly, so the button start function only for "switching on" mode, cause strength of spring is not enough to put the button to "off" position.
you should "align" the spring and the button will work properly, actually, if I remember correctly, you should bend the spring a bit, cause after the pressure which caused malfunctioning it became too flat.