2005 Nissan Hardbody Can't select gear

Hi I have a 2005 Nissan hardbody 4x4 2.4 16v
I changed the clutch kit about 20 000 km ago and yesterday it just doesn't want to select a gear had a look at the master cylinder and all looked fine when I switch off the motor I can select but when it's running can't select and clutch also no resistance could this be the thrust bearing?Any help will be appreciated want to find out before I take that gearbox down coz that's a hell of the mission

Thanks allen dog