Unusual 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

Welcome to APUG!

Is there a masking accessory in your light path?

By the way, it is not necessary to post your enquiries in more than forum - most people who read APUG sort threads using the "New Posts" function.

I am sure someone here will be able to help.