I disassambled/assambled rear wheel hub, but now rear wheel is not turning freely, what have i done wrong?
You likely tightened the cones too tight. Read up on the proper procedure for cone adjustment and be sure to grease it up well.
There are unfortunately a dozen different things that can go wrong. First on the list, as mentioned, is over-tightened cones. But beyond that it's easy to get a spacer in the wrong place to that, eg, the freehub rubs against the hub body, or you can simply get a foreign object (eg a washer) jammed in between components somewhere.