First, try the shutterrelease button.
It could be that the camera is prereleased (side button below wind crank).
If that doesn't help, remove the magazine, and push in the lower of the two doors you see in the back (don't worry, you'll do no harm that way).
You'll see a slotted axle head below the lens. Insert a small screwdriver, through the back, into the slot, and turn it clockwise until it locks.
Sometimes, turning the thing a little bit will free the camera. If you hear a click and feel a sudden tension, remove the screwdriver before turning the axle until it locks, and you'll be able to use the wind crank to recock the thing again.
But mind you, if it doesn't you'll still feel tension (you're winding a spring by rotating the axle).
Now you should be able to remove the lens (the back is already off).
Do so, and 'excercise' the camera a few times (press the release, recock). If nothing out of the ordinary happens, reattach the lens and do it again.
If still nothing untowards happens, you'll be fine. No worries.