Worst case: you will have to take the extension tube apart, starting with the screws that hold on the rear bayonet plate.
Can you describe what the state of both lens and tube is?
It may be possible to cock both by simply inserting a coin into the slot of the axle at the rear of the tube, turning it clockwise until it locks, or if it does not lock, until the slot is horizontal.
If you can get that far, you will need the help of someone. While you keep the axle from rotating, still using that coin in the slotted end of it, your helper can depress the lens catch and remove the lens.
If that doesn't work, it's disassembly time: the tube will have to come apart.