Gearbox/Transfer Box Issue? Please Help

Hi all!

After owning my Defender for a few months the inevitable has happened and I've encountered my first MAJOR issue. My transfer box doesn't seem to be engaging correctly in any gear, when attempting to drive it feels as if some of the cogs are slipping and the car is making a chugging type noise from under the cab whilst doing so. Any advice on how I can diagnose the issue would be great, thank you

Probably want to let people know a bit more info regarding the vehicle?

Engine, gearbox etc?

Gearbox mainshaft could have stripped its splines? Transfer boxes can strip gears,

Have you opened the inspection cover on the tb?

Are you able to get it into gear and drive at all? Has the diff lever been knocked partially into neutral?