Although it's more likely to be hydraulic, this could also be mechanical. I have seen a pressure plate finger break. It will jam up the clutch and keep it relased, then fall out of the way for awhile and let it work pretty well.

If the clutch pedal feels normal with the engine stopped, you probably need to pull the transmission and investigate. I know this is not what you want to hear. I hope you're not one to shoot the messenger.

Hope this helps,