Assuming there are no bits broken off and as most mirrors are designed to come off when knocked its usually just a matter of lining up the pins and holes in both parts and giving the mirror a hefty shove onto the housing, the parts should click into place.
Thanks grouse, but it was broken totally beyond repair. Needed to buy complete new mirror assembly. My wife wishes she could have caught the little ba***rd who kicked it. Joys of motoring?