Drain the basin on the machine as you normally would and remove any standing laundry if applicable. If the agitator got stuck mid-wash, wring out your laundry to hang it up to dry so you can focus on the machine. Feel underneath the bottom lip of the agitator and try to pull it up; if there’s considerable resistance and the agitator doesn’t pop out, your particular model may have a holding screw anchoring it in place.
Look underneath the outside of the basin to locate a rather large wing nut threaded over a bolt. Loosen the nut -- use a wrench if the nut sticks -- and carefully pull down on the assembly to remove it from the basin. From here you should be able to look upward into the basin, underneath the agitator, and find the holding screw. Use a flathead screwdriver to loosen the holding screw and remove it so you can remove the agitator.
Inspect the agitator once you have it out. If there’s wear on the underside, the agitator is making contact with inside of the basin as a result of an unstable balancer, a smallish metal or hard plastic ring that rests inside the upper cone of the agitator. Fish out the balancer with the screwdriver; if it’s busted or worn, purchase a replacement and reinstall the new balancer according to the manufacturer’s specifications.
Check out the upper cone of the agitator and look for cracks or splits, either of which will cause an imbalance or the agitator to not spin, just like an unstable balancer. Depending on the severity of the damage, a thin application of industrial strength adhesive gel may do the trick. Apply a thin layer over any splits and squeegee over the gel to ensure it closes the crack. Wait a few minutes before reinstalling the agitator and attempting to run a load through a cycle.
Sand away any rusted or rough spots with steel wool if you’ve purchased the washer second hand and it’s been sitting unused for some time. After removing the worn spots on the shell of the agitator, wipe away the dust, wash it with warm water and reinstall it.