Dell 3100cn problem

I have a Dell 3100cn, which has been a fantastic printer for me so far, but has finally given me a problem.  I had to put in a new cartridge the other day and all went fine closed back up and powered up and now I get a message that the "Top panel is not closed". Of course it is and I can find nothing that even resembles  a contact switch or the like.  I would hate to think my printer  is gone because of such a message.  Any ideas would be appreciated I have tried and looked all I can.  There seems to be no service manual for this printer so even that resource is not available,  Thanking anyone in advance.