Error message:
Unable to print
Troubleshooting and routine maintenance chapter in user's guide

I cleaned the unit as per user's guide, but no change

Have you tried printing a "test" sheet using only the printer? Also you might need to change the printer cable if its not communicating with your computer.

Brother Solutions Center: MFC-640CW: Manuals: United States (http://welcome.solutions.brother.com/bsc/public/us/us/en/doc/manual_index.html?reg=us&c=us&lang=en&prod=mfc640cw_all&dlid=&flang=English&type2=-1)