Who conquered Egypt during the middle Kingdom?

The Hyksos did. A near Asian tribe who introduced the composite bow and wheeled chariot to the Egyptians.

http://en.wikipedia.org/wiki/Hyksos

Note: Hyksos was merely the Egyptian phrase meaning "foreign rulers", and some scholars believe that some of these people were actually the tribes of Israel as the Biblical account attests. The best estimate for the israelite migration to northern Egypt is between 1900 and 1800 BC.