Cordless Drill: Chuck is Stuck Open or Closed

The drill chuck holds the drill bit in place with jaws that firmly clamp down on it. If the drill chuck is seized open, or closed, it will need to be repaired. If it is seized closed, you can release the jaws with channel lock pliers. With the drill bit removed, close the chuck and inspect the jaws for alignment. If they are not even at the tip, you can gently tap them in place with a hammer to realign. This should solve the problem. However, if the problem continues, replace the drill chuck.
If the chuck is stuck open, check that the screw inside the drill chuck is fasten tight. If it is loose, it can prevent movement of the teeth. To solve this problem, insert the proper screwdriver in the drill chuck and tighten. Remember, this screw is reversed thread so turn it counter clockwise to tighten.
If you determine that the drill chuck needs to be replaced to solve this problem, begin by inserting the proper screwdriver into the chuck to remove the screw at the bottom. Snug the chuck against the screwdriver and loosen by turning it clockwise (this is reversed thread). Once you have broken the tightness of the screw, loosen the chuck and continue to remove the screw. With the screw removed, insert a big allen key in the chuck and tighten firmly. Now, insert a pipe over the allen key for leverage and turn counter clockwise to loosen the chuck. A second method is to hit the allen key quickly with a hammer to loosen. Once loosened, continue to remove from the spindle.
Install the new drill chuck by threading it on the spindle and then insert the allen key and tighten the chuck on the key. Use the key to tighten the drill chuck on the drill, then loosen the drill chuck and remove the allen key. Insert the screwdriver inside the drill chuck to tighten the chuck screw, remembering that the screw is reversed thread.

If the gears are worn down, this would cause this symptom. If the motor spins, but the drill chuck does not or only works in certain gears or clicks like it is jumping gears/gear teeth, then you will need to replace the gear case.
To replace the gear case, first remove the end cap on the drill and then release the brush springs. Insert a screwdriver into the drill chuck and remove the screw by turning it clockwise, remembering it is reversed thread.
Next, insert a large allen wrench inside the chuck and tighten the chuck down on it. Hit the allen wrench with a hammer to loosen. Before doing this, make sure the transmission is set to the lowest setting. Remove all of the screws on the cover that secure it to the housing as well as the screws securing the transmission clutch assembly. Remove the cover and transmission clutch assembly and pull the gear selector switch out. Remove the motor from the transmission and insert the motor into the new transmission.
Reinstall the gear case and motor back in the housing, aligning it up properly to lock into the housing. Reconnect the gear selector switch and then the cover back onto the case, ensuring all the wires are tucked inside the channel in the housing. Reattach the transmission clutch assembly to the case. Snap the brush springs back in place and reattach the rear cover. Reinstall the drill chuck hand tight and install the screw back inside the chuck remembering it is reverse thread (turn counterclockwise to tighten).

The key is inserted inside a hole on the side of the drill chuck, which locks into a gear on the drill chuck. When you turn this key, it will open or close the drill chuck. Make sure you are using the correct size chuck to lock into the gear to allow the jaws to open or close.