I'm fairly new to guns and am not sure of the correct terminology but I just picked up a Model 81 the other day and had the same problem--brought on by myself.
Assuming the action is clear of any interference, most likely the hammer (looks like an acorn at the rear of the bolt) is out of sequence.
The process might take some force, but with the barrel of the rifle against the floor, push the bolt handle forward into the gun until it is close to locking down (DO NOT FORCE HANDLE INTO LOCKING, EVEN THOUGH IT IS CLOSE TO GOING!) and then "TWIST" the acorn clockwise until the pin on it "snaps" forward into the lower channel. Make sure there isn't a bullet in the gun, seriously. Don't use tools or anything that will chew up the hammer. There is a small pin on the acorn--that's all the leverage you have to work with. Again, with the bolt pushed forward twist the acorn clockwise until it seats and you should be done.
You might lose some finger skin, you might scare yourself, and you might not even be able to twist it by hand but I was able to oil up my bolt enough to make it go back in order fairly easily by hand.
Once the hammer (acorn) is snapped back into place, the action should be in correct sequence and you should be able to cycle away at will. If you need to remove the bolt out to figure everything out, just pull the trigger beyond firing (extra force) while pulling back on the bolt handle and it should slide out, creating a whole new level of confusion if you don't know what you are doing....
