You're talking about the overdrive lockout button, not the shift indicator. First of all, if you've never used this feature before, press the button on the gear shift knob to see if it turns off. You might have just accidentally pressed the button.

What the OD lockout does is sort of counterintuitive, especially with the upward pointing arrow on the light that comes on in the dash. It's actually the reverse. When you press that button and the arrow light comes on, you are actually preventing the transmission from going into the final overdrive gear. You would use this feature to maintain better control and prevent unwanting shifting in certain circumstances, most commonly on mountain roads and other hilly driving, and also when you're towing this feature is useful.

If pressing the button doesn't make the light go off and give you full use of your overdrive again, do a search for overdrive relay. That's the usual cause of problems. Also check the 700-900 for overdrive issues, since it is the same information as applies to your car.