gear box trouble

i have a 1974 sereis 3. it has been running fine untill today. i reversed onto my drive last night and parked it up. i turned of the ignition and then accidently knocked the red lever down so i just pushed it back up. I came to use it today and it wont select any gears while its started, almost as if the clutch isnt working, i tried starting it in gear but it wont start when its in gear only in neutral. I have pressed the yellow knob down and tried to pull the red know down into neutral and then down into low ratio to release the yellow knob but the red know will not move, its as if it is stuck in high.

i did manage to get it into reverse but it still moved back even with the clutch depressed.

any help or suggestion on how to get my gear box working again would be helped.

thanks