There could be a few reasons for this. First of all check that the pedal is free to go all the way to the floor. Does it return or does it stay flat? Check the fluid in the master cylinder cylinder - does it need topping up? Is it leaking? If all looks well there, then it could be that the seals on the slave cylinder have failed - effectively forcing pressure past the seals and seizing the clutch. If everything seems well, then it could be the clutch release arm.