It is likely that the Printhead has gotten dirty.
(1) Turn off Label Printer and allow it to cool down.
(2) Open the Cover and use the alcohol pads to clean the Printhead and Platen Roller. (Any 70% Alcohol pad would work; These are commonly available in pharmacies and grocery stores as well as on Amazon.) 
Allow 2-3 minutes for components to dry, then close the Cover and turn Label Printer back on.
NOTE: Firmly press the Cover to confirm that it has been completely closed.