Three possibilites...

1) You've pulled the bulb partially out of its socket and thus, it no longer lights up
2) You've damaged the bulb and need to replace it (unlikely but easy to do)
3) The bulb has simply come out of its holder (a small appropriately shaped plastic mount that the bulb mount rotates and locks in...as a result, the light is not shining through the translucent cover....

...my money is on No.3