Fountainhead

Hi,

If you're at the track and this happens, could be boiling fluid due to heat. The clutch line passes directly past the exhaust. When the car cools off then it will drive normally. Changing to higher boil point brake fluid (clutch uses brake fluid) will usually correct this, brands of "best" fluid are legion, read up on them.

If you're driving around in traffic and it happens, could either be:
A. Master cylinder failing (no leak and no pressure but can pump back up and no fluid loss in clutch reservoir).
B. Slave cylinder failed (leaking fluid because the seal on it's gone out) you'll see it dripping out of the transmission bell housing bottom, and your clutch fluid will be low.

Those are the two most likely scenarios I've seen on here.
If the Slave Cylinder is leaking I would have them make sure the clutch pressure plate assy doesn't have fluid on them, it ruins them over time.