Printer "imageCLASS" MF4350d. Now can't print/copy ... Says: "LOAD PAPER CASSETTE: LTR"

[zpxykm]

   My printer is "imageCLASS" MF4350d and has been working very good for years.

    Now it doesn't do  print/copy ...any more and says: "LOAD PAPER CASSETTE: LTR"

    Yes, neither copy or print, at begining the sound/phenomena is normal, but soon changed 

to alarm sound and the red light turning on.

    What wrong is the paper cassette causes it doesn't copy ....    ----Zhao