If it was the clutch, slave cylinder etc, i would have thought the gears would engauge easily but crunch as your not seperating the engine/trans. Not being able to select it at all sounds like the selector has packed up, not sure about your particular gearbox, but mine seems too "thin and weedy" to do the job it does.
Probably a mixture of your clutch going, in time with something else. Is it easy to select first, for example your going to pull away so instead of going straight for first, go into second then straight into first it should be easier? Does your second gear crunch when going above 20? (downshifting for corner/junction etc).
I notice, and i did this by accident, i can take it out of third gear without using the clutch and nothing happens at all :S fiats are strange lol.