All backlighting on the stock gauges are created via bulbs. there are several bulbs to illuminate the entire gauge face. Your bulb is likely burnt out on that section. Remove the gauge face and surround then replace the bulb. While its out you may consider an LED replacement mod. You can look in the Aesthetics forum FAQ and also search in there for instructions on removing the gauge face (both to replace the bulb, AND to do the LED mod...)
Correct.. I was not meaning that you had to SEPERATE the gauge and glass.. I meant to remove the guage/faces... I guess that was a really bad choice of terminology.