The speed (1-2) Switch is either stuck or won't change speed of rotation.
Using a Torx T20 screwdriver, remove the four screws attaching the back plate.
Remove the five screws attaching the drill handle.
Remove the 4 screws attaching the front of the casing.
Locate small metal bar above plastic part of drill.
Attempt to move bar back and forth. If it wont move, try to move it while rotating the chuck. If it still will not move, there is a problem with the gears. For help see the Planetary Gear Replacement Guide
If the bar moves, next check the small plastic switch that is numbered 1 and 2.
Make sure the small tabs on the underside of the switch that move the bar are intact.
Replace switch as needed.