look under the bonnet, on top of the gearbox, is there a black cable sheath(about 10mm diameter) running to a large long lever arm on the front side of the gearbox?
Or is there a rubber pipe, with a coil looking metal pipe, running into a mslave cylinder on top of the gearbox?
this will tell you if its cable / hydro??
If its cable, possibility of stratched / nearly snapped cable? Could be a faulty pedal bracked maybe??
If its hydro, could be system needs bleeding / has a leak? Again, could be faulty pedal bracket? Faulty slave cylinder?
In both cases, i have seen the actual clutch release arm inside the grabox fatigue and crack over time, and then bend internally, and not work correctly? though this is a last resort, check everything else first, as this means gearbox off??