The shutter button can be rotated in the "down" position to lock the shutter. This is of course only useful to hold it open on "B". Try rotating the serrated bit of the shutter button a little bit to see if it comes up again.
What lens? Is it a 50mm or any other lens?
If it's a 50mm, it should be attached to the inner bayonet. Set the lens to infinity (for convenience reasons only), push down gently on the spring lever on the inner bayonet next to the infinity position to release the lens' locking pin (should be painted red), and rotate the lens clockwise out of the bayonet lock.
If it's any other focal length, it should be attached to the outer bayonet. Set the lens to infinity. Push on the locking lever on the lens' mounting flange to release the bayonet "claw". Grip the lens by the mounting flange and rotate the entire lens clockwise all the way until the red dots on lens and body line up. Then you should be able to take the lens out of the bayonet.
The Contax/Kiev double bayonet mount is a bit awkward, but actually quite a clever construction for the early 1930s. At that time everybody else had to screw their lenses in and out. 🙂
Philipp