DIAGNOSIS The light timer setting has probably been maxed out or set to "always on." It's an easy setting to change accidentally when programming remotes or using the wall console.
FIX On the back of the motor unit, there's a small rectangular case for the light bulb. Next to it, you'll find two small slide switches. One controls the auto-light duration. Slide it to a position other than the far right (which is the "always on" setting). The middle positions give you 2.5, 5, or 10 minutes of light after an operation. Just moving the switch should fix it immediately.
IF THAT DIDN'T WORK If adjusting the slide switch does nothing, the logic board's relay for the light circuit might be stuck closed. Try unplugging the opener for a full minute to force a hard reset of all electronics. If it comes back on and stays on, the board itself may be faulty.
HEADS UP That always-on setting is useful if you're working in the garage, but it will significantly shorten the life of the incandescent bulb. Consider swapping to an LED garage door opener bulb if you leave it on a lot—it runs cooler and uses less power.