Please check the following points:

• Identify the missing color and put in a new toner cartridge.
  To see how to Replace a toner cartridge, click here.
  
  
• Clean all four corona wires (one for each color) inside the drum unit by sliding the green tab.
  To see how to clean the corona, click here.
  
  
• Make sure the corona wire cleaners are at their home positions.
  
  
• Put in a new drum unit.
  To see how to replace the drum unit, click here.