Not too sure why it would suddenly stop working - anyway:
May I suggest you try a reset of your remote:
Press & Hold the 7 & 9 until the remote's light (red LED above the SKY logo button) flashes 4 times
Then press 1 & 3 together to go through to re-pair with the box and your TV - make sure it has line of sight of the box as until it is paired it uses Infrared
