Is it jammed, (ie. you can't move it at all), or will it not engage, (ie. it moves down a little then jams)?
If the latter, put the bike into gear and roll it back a little to move the gears, then put it back into neutral and see if it engages. If it does you can continue to go through this process until you get bored of it and repair the kick start mechanism, (can be as simple as removing burrs from the teeth).
If it's totally jammed you will need to remove the gearbox outer cover and investigate, easy enough to do in situ.