Basic operation

[KT50]

Ionic suddenly stopped lighting up when I move my wrist. 

[DramaQueenDiva]

I would try a soft reset initially by holding down the left and bottom right buttons for about 10 seconds.  Once the Fitbit logo appears, you should feel a slight vibration signaling a successful reset.

If that fails, I would try a hard reset, which is basically the same procedure as the soft reset, but instead of holding down just the two buttons, you hold down all 3 buttons.

I would also consider putting it on the charger for a little bit before the resets, just to make sure that there is enough juice in it to light up when you move your wrist.  That may be the issue, and if it is, then there's no need for a reset.  It should take about 2 hours to charge.