My schlage door knob lock tumbler stopped working the key wont unlock it anymore

If the cylinder in the lock is not turning anymore when you put the key in -- then there could be a stuck pin inside the cylinder. use a teflon or silicone based lubrication and spray into the key hole and cycle the key in and out a few times and then spray some more lube in and cycle the key some more,, spray one more time and let it sit for awhile --- after sitting for awhile, try the key again.
if it still wont turn the cylinder then it needs to be rebuilt,, take the lock down to your local locksmith shop and have them rebuild the cylinder to the key you want. the cost of this is only about 8.00 to 10 dollars ( depending on where you live ) and it only takes about 5 minutes.
now there is also a chance that the lock might have another issue going on here --- if you remove the knob from the door and with it in your hand, try the key and see if it turns left and right,, if it does turn then the problem is not the cylinder but the thumb turn part of the lock jamming up,, you can also take the parts to the lock shop and get a replacemnet pretty cheap.
one more thing to check is to make sure the lock is not to tight on the door -- and what I mean by this is that on some hollow doors they get squished in to the point where the lock/unlock part of the lock drives into the thumb turn part and it has way to much pressure on it. again - if you remove the lock from the door it should all be pretty clear what is the problem here.

c...