(1) Replace the battery.
(2) Verify that the power switch is functional. Separate the frame from the body and examine the surface of the board. The power switch at the top-right of the board needs to look EXACTLY like the programming switch in the bottom-right. Oftentimes the small, white, raised button will break off the switch and prevent the button from working. The cover of the switch can also fall off and prevent it from working.
If the switch is damaged, you'll need to get it repaired. Send it into SP, Ydna, or someone else qualified.
(3) Check the main capacitor for damage, often caused by grip panel screws. If damage is apparent, the capacitor or entire board would need to be replaced. This requires moderate/advanced soldering experience to do yourself.
(4) Plug the battery into the connector and push the power switch manually using your finger. If the red LED lights after approximately two seconds, then the problem is you're not pushing the external ION jewel hard enough to turn the gun on. Reassemble it and try again. If the red LED doesn't light, there is an electronics problem and the board will need to be repaired or replaced (send it into SP).
Let food be thy medicine and medicine be thy food