Car will go into gear while off but not when running. Clutch problem?

Hello I have a 1987 vw cabriolet, all original parts. I just replaced the clutch cable 2 months ago. Today I drove to work normally but when I tried to go home here is what happened:

Started it, tried to put in reverse and it just jumped violently, there was a loud CRANK but it did not go into gear. Actually, it just would not go into ANY gear.

I Turned it off, checked the clutch cable, the clutch lines. Everything looked normal. Went back inside, and still could not shift. BUT. I tried to put in in gear with the engine turned off and it actually did. So I left in in gear and started it but the same thing, it just jumped violently but would not start.

NOTE: I've never put any clutch fluid. Actually different people tell me different things, some tell me that It does NOT need or require fluid, others tell me it DOES. Also, i've never put transmission fluid in it because I cannot tell where to put it.

I don"t get it, it was just fine this morning when I drove it to work. Except for a leak in the coolant flange and I did not drive it or even started it until after work