is it a GREENSTAR 28i junior? [red & blue light] 
you mentioned the FAN was working 
does the boiler SOUND like a kettle boiling when trying to start up? 
IF YES then condensate could be blocked 
if external [of boiler] condensate pipe blocked then there are plenty of YOUTUBE guides of HOW TO UNBLOCK FROZEN CONDENSATE PIPES
                
 Q
                
 February 2015