I have a 1996 mercury sable. the inside of crankshaft pulley is spinning but nothing else, What happened ?

It's like rick29148 said, except there's a rubber type piece in between the inner and outter parts. Sometimes they disintegrate and let go. Back when people used to check the timing on their engines, the first sign would be they would notice that the timing would read way off, though it actually wasn't, and the engine would run fine. Sounds like yours is totally shot. You will have to have the old one pulled off with a harmonic balancer puller, and a new one installed. Be wary of used ones from the junkyard, as they may already have the same problem, or could develop it soon thereafter.