Hi guys.
Newbie to the site and thanks for the add. Been riding and racing (offroad enduros) for about 43 years. 1 issue to check on shifting issues if you suddenly have the lever not go in gear etc or it gets "stuck" . You may have to remove the side case, remove clutch basket etc to gain access to the shift star mechanism that turns the shift drum. A bolt holding this star may have backed out or spring broke. Usually the shift shaft crosses over into clutch area and is accessible. Check the schematic on this and keep us posted.