Pull the valve cover and take a look at the valvetrain. A rocker may have broken and is jamming the cam or gears.
If you can spin the engine with the cam chain disengaged it would suggest the bottom end is not the problem - which is good news.
When you start to put things back together you will need to make sure the cams are in the right place because you have changed their relationship to the crank by turning the engine over with the chain not engaged.
This thread may help -
https://faq.ninja250.org/wiki/What_i...t_procedure%3F