Most likely it is a dead battery simply replace the battery. If the light does not come on you may have a faulty LED and will have to replace the LED.

First ensure that the mouse is on if not press the restart button on the bottom of the mouse to turn it on. If the problem still persists try restarting the computer if that does not fix it you will have to clean the scroll wheel or your scroll wheel is broken ad you will have to replace it.

Perhaps try restarting the mouse using the button on the bottom of the mouse. If that does not work make sure the USB receiver is plugged in firmly or try moving the receiver to a different USB port. If the problem continues then restart the computer.