I don't have that model in front of me right now, but I found this advice to resolve the issue.
* Open the back cover* Pull the flap down that hides the orange roller on the fuser (Careful it's Hot on that fuser!!)* Locate the thin piece of plastic that runs from the centre to the right hand side. This bar is the problem* 3/4 along the right hand side, there is a small bit on this bar that should sit in a hole.
This "little thing" is not on the end of the horizontal rod -- it's indeed in the middle or 2/3 of the way down, starting from the left, traveling right along the rod. This tiny little perpendicular appendage (about 1 mm? 2mm?) points "down" and on my printer at least, was out of a rectangular trough hole that travels parallel to your line of sight as you're looking straight into the back. The appendage needs to be placed back into this rectangular trough.
This protusion is just one possible solution to the problem. If you are not able to fit it into the "hole", or "rectangular trough" and resolve the cover closed problem, then it could be a sensor that has malfunctioned. I do not have a picture to show you unfortunately, there shouldn't be that many different places for it to go.I was hoping your issue could be resolved by that simple tip. If you are sure no paper could be jammed anywhere and the back cover issue persists then I am afraid you will need to get your printer serviced by brother technician to replace the sensor or determine the problem with the machine.