Clutch wont disengage 250 kx 250

Here's how I'd try to fix the clutch:
1.) Try to adjust the cable at the handlebar; usually there's a little bid of clutch lever travel (like 1/4"-1/2") before the clutch disengages. For right now, adjust the clutch so that there isn't any play and the adjustment all the way out. If the clutch disengages the transmission, then you know you have to fiddle with the cable to get it adjusted properly.
2.) The clutch plates could be sticking: here you'd have to take the clutch cover off and get to the clutch. Remove the clutch (usually an impact is ideal for removing the clutch holding nut. Take the clutch apart (steels and clutches), make sure they aren't sticking together. Then dip the clutches in oil, then reassemble.
3.) Sometimes the clutch basket (holds all of the clutches and steels in a "basket") gets grooves in it. These grooves prevent the clutch from disengaging. Sometimes you can file the clutch basket fingers down to remove the grooves. Alternatively, if the clutch basket fingers are really worn, you may have to replace the clutch basket.
Typically, when your in the clutch side of the engine case, I'd examine 2 and 3 (above) at the same time.