HELP! on 2005 Mini Cooper S

I have 2005 mini cooper s convertible manual 6 speed transmission. I am not able to change gears while car is running. When car is off it will go into all 6 gears. what is the problem????

Asked by for the 2005 Mini Cooper S
OK, first thing first. Is there fluid in the clutch master cylinders reservoir?
Did you have any problem with the clutch before, was it noisy?
How many miles are in the car?
It seems to a be a clutch problem. Could be the master or slave cylinder, fluid leak, worn disc or other internal clutch problem.