Ceramic ring installed wrong and now jammed
Some fixtures are sold with a special tool for tightening those rings. You may check with lighting store to see if they have one. If you can disconnect the fixture and take it in the the store with you they may be able to get it off with your having to purchase one. It looks similar to this  

This image is a "Spectrum G9 Lamp Shade 20mm Locking Nut Ring Removal Tool" and was from amazon but listed as unavailable.