Armature won't respond to keyframe

[eurosoul]

Hi Blender Artists

I had a headache because of this problem , I don't really know what have i done to make it like this
The problem is when i was animating a character suddenly i found one of the bones not responding to key frame
when i pose it , it stays at that pose and won't respond to any keyframe on the time line
I looked at that bone closely and i found it dark blue in wire frame mode while all other bones are black
Also in graph editor all the curves were colored except the curves of this bone were only grey !
please help me , what mistake have i done and how to fix this ?

[eurosoul]

The dark blue color isn't part of the problem i think
that color change was just because i deselected it