Blades won't spin on Craftsman II riding mower

When the blades are engaged do they turn? It could be a belt is loose tha causes the blades not to spin. Check the handle that engages the mower deck to see if it goes into the proper slot which would cause the blades to spin.

Your belt may be on wrong or slipping.

the clutch might have failed

First check to make sure you can turn each blade by hand, with the mower off. If neither blade spindle is frozen remove the deck and make sure all the pulleys move freely and function like they should. The cable should have been operating a spring loaded pulley arm that applied tension to the deck belt to make the blades function. Some times build up with block that from moving or rust will make it stick.

just replaced some pulleys on my craftsman 42 inch cut and now the blades are ingages all the time, but it will start

That is because you need to hold the blade(s) still when torquing loose the blade bolts. Some of those are nasty to "crack" loose. But with the proper "6 point" socket (12 point is not recommended at ALL!) and leverage? Piece o' cake! Also craftsman does make a special tool to lock the blades down to remove them. There are also generic ones for riding type mowers (I've seen them at Lowe's) blade removal. Preventing the crankshaft from spinning is paramount to getting the blades off (Not to mention safe?) It depends on your level of mechanical expertise. Experienced? then you can do it, Not experienced? Go get the special tool to prevent the blades from spinning. Life is so much easier when you got the right tool for the job!

sounds like either the spindal bearings are shot or maybe the spindal shaft is bent. check for up and down slop at the blades. then turn blades towards each other and check that the heigth lines up. also chec the idler pully to make sure it is not slapping around.

You might want to check this may sound silly but check to see if the blades were put on upside down.

It sounds to me that you need a new mower deck drive belt, or your blades need sharpening. If the blade edges are rounded, they won't cut butter... Also, when mower is off, check mower deck drive pulleys to make sure you don't have any that are locked up or are dragging. You can check this by pulling the belt away from pulley and spinning the pulley by hand. MOWER MUST BE OFF before attempting any of these checks. You would be surprised how many people would try this with the mower running.
Hope this helps

have you tried the safety switches?