Q: Unable to shift while the car is running. Able to shift when the car is off

I have purchased this car 10/20/2016 and have driven it since. I have not encontered any problems with the clutch during all these weeks. No slipping, shifting always smooth. Nothing to indicate anything was wrong.

I stop at a red light, put the car in neutral. Light turns greens and just like that THE CAR DOESN'T SHIFT IN 1ST. Nor 2nd... Or any other gear. The clutch pedal feels weak with almost no resistance.

I check the fluid, and CANNOT find a separate reservoir for the clutch. I come to the conclusion that the clutch fluid runs from the break fluid reservoir. It's full, but somewhat DIRTY.

I turn the car OFF, and I am able to shift in all gear.

With the car OFF, I put it in first. I try to start the car. It attempts to start and Jerk forward violently, but NEVER actually starts.

I call the tow truck and get it to my mechanics. My mechanic claims he is sure I need a CLUTCH KIT without doing much inspecting. Can this simply be a problem with maybe one of the cylinders

My car has 100000 miles.
My car has a manual transmission.