Does the clutch pedal have any weight - i'm thinking maybe the release bearing/slave cylinder has stuck in the 'clutch pedal down' position - but this would be very unusual! Or maybe the clutch itself has seized 'open'.
Does anything happen at all when you lift the clutch in first gear as if you're about to pull away?
You can normally feel (to a lesser or greater extent) the gears engaging through the gearstick when you move it from one gear to another - do you feel anything? Does the gearstick return to the neutral position naturally when you move it out of gear? Is the gear linkage complete and attached correctly?
Assuming there is no sudden failure of a gearbox component (usually peceded by nasty noises), I reckon you've most probably got a clutch or release bearing problem. Is there any sign of hydraulic fluid leaking from underneath the gearbox? This would point to failure of the release bearing, but this would normally cause the clutch pedal to travel to the floor with little or no resistance.
I've never known a gearbox fail without any warning. Thinking maybe the input shaft has snapped, but I'm sure you would have noticed if that had happened!
Think the gearbox is going to have to come off for you to be able to identify the problem!
I would think that given all of the above info that the gearbox first motion shaft has sheared. (shaft from clutch into the gearbox) box has to come out to check this.:turd
PS sorry I should have read above reply properly before answering.