Chances are the EL (electroluminescent) lighting element in your boost gauge has gone bad. This is something that can/does happen on the lunar series. I would take the above recommendation and contact Autometer for a replacement.
I have seen a few come out of the box with the lighting portion inoperable.
Similar, but not exactly like it. It's not worth it to fix it, as you'll tear up the gauge getting it apart (experience speaking here). The bezel is crimped onto the white gauge body, and is so soft that any attempt to uncrimp and reattach will most likely be botched (experience again).
Even if you don't have any luck with Autometer, a local dealer might be willing to warranty it for you. Couldn't hurt to try a few of them.