This could be caused by any of the below mentioned things:
Paper pull-in roller holder
Base pad in the paper tray
Jam clear cover
PF-related gears
Main PCB
Paper feed motor
Problem here is that we do not know what part broke and fell out. Amazingly enough I found the service manual for the device and you can download it [http://www.keepandshare.com/doc/3015160/mfc845cw-pdf-july-29-2011-9-23-pm-20-1-meg?da=y|here] I do hope that this will help you out. Good Luck to you and your printer and let us know how it is going.