-W984 drill 14volt (the label has worn off) I opened up the chuck to put in a large bit, but now it is stuck fully open and I cannot close the chuck, please can you help.

Put some penetrating oil on the chuck and let it soak overnight, they should come loose.

Hold the chuck and run the drill either forward or in reverse until the chuck is fully closed or open then reverse it while holding chuck.

The large black part of the chuck should turn left and right to extend (close) and retract (open) the three chuck teeth that hold the bit in place. Turn the chuck to open it far enough to get your bit fully in the chuck then turn it closed to hold the bit in. When the teeth make contact turn the chuck a bit further to get it tight enough that the bit won't slip. When you get used to it, sometimes it's easier to hold the chuck lightly and turn it by lightly squeezing the trigger to turn the chuck with the drill motor.

If the chuck is stuck open and you cant "muscle" it closed something is probably jammed up in there and it needs to be replaced. Chucks take a good share of the abuse on power tools and most manufactuers consider them a "common wear item" like brake pads or Tires on your car. A chuck is fairly simple to replace if your drill stuck open and they are commonly replaced on the higher end "Industrial Grade" Power Tools like DeWalt and Milwaukee.
Looking at the Make and Model of your drill it would probably be easiest and most cost effective to just replace the entire drill. If you tried to replace the chuck and found a chuck that will fit your tool it may cost you anywhere from $25 - $40. For Around $40 you can purchase a new drill with a fresh battery.

It's just jammed in fully open position, take a set of channel lock pliers around the chuck and unstick it .

Hello. Wrap a towel around the chuck and use a pipe wrench or a pair of channel locks to put a little force on the chuck. J.

If you mean the chuck jaws seem stuck open, spray around them with some penetrating oil or spray like WD-40, RP-7 or similar and let sit for a couple hours... Then you can tap around the chuck body with a light hammer or spanner etc... just lightly tap... Then if the chuck has an arbor lock to prevcent it from spinning, be sure that is engaged then try to release the chuck jaws. Sounds like they are stuck open through tight friction or rusting on the inside edges (that you dont see). Be careful not to leave them in the fully open position when stored as this is exactly what can happen!

hold the back of the chuck with channel locks or a vice and use the same channel locks on the front and open the chuck manually it should work and you can get to the screw to replace it or the hand chuck. this can be a little frustrating but hang in there and you'll be a pro shortly.

I'd put penetrating oil in it and tap the chuck with a plastic hammer. That should free it up.

Best rgrds, W/D

this how to get a chuck off a drill.look inside the chuck for a screw. it will be a left handed screw.once you get the screw out put the chuck in a vice . you will need to squeeze it very tight. then put the drill in reverse and pull the trigger.if i wont come off you will need to hit the chuck with a hammer while you are holding the trigger. if you have a bit stuck in the chuck and cant get it out put the chuck in the vice and hit it very hard with a hammer until the chuck falls apart.i work on drills every day ,this is the only way we have found that works