NP 4050 -

The door may be closed, but there is a inter lock switch that is not being closed. If you have all four paper tray, try to turn the copier off and disconnect the bottom two cassette's from the back of the copier and turn it back "ON" and see if the copier works. If it dose you have a problem with the bottom two tray's. You have a very old copier, can you do with out them? If not the switches are toward the front & it may just be knocked out of place... Good Luck...