Slave or master cylinder failed, or
oil leak is keeping the clutch plate, pressure plate, and flywheel
connected.

Check the power steering fluid level.  Look under the car for a big puddle
of red fluid from a broken clutch line.  Look at the slave cylinder while
someone presses the pedal and see if it's actually moving.

If it happened abruptly, it is more apt to be a hydraulic issue than a
mechanical issue with the clutch itself.  But a sticky clutch is possible.
--scott

Check fluid reservoir - if low - refill.

Might be more complex than that - leak, need to bleed etc.

It's possible that the clutch disk is simply 'stuck' on the transmission
imput shaft and won't slide away from the flywheel when you depress the
clutch pedal.  I had this happen to me on a Toyota pickup once.  The fix:
Put the transmission in high gear with the engine stopped; push in the
clutch pedal and start the engine.  This should break it loose.