problem with durst m805 color

I just bought a second hand durst m805 color enlarger with a good price. when i put neg in carrier, i wondered what density screw will do and what exactly will change in image, when i was turning it suddenly the image which comes out through lens went out, the lamp is on, I'm sure about it, but no light passes through lens, as if lamp is off!! I don't know what's wrong with enlarger, help pls