You are required by law to have two rear view mirrors in total. one of them has to be the drivers side "wing mirror" the second one can be either a passenger side "wing mirror" OR the internal rear view mirror.
So it depends on which mirror you've had knocked off.
If it's the passenger side, then as long as you have a usable internal rear view mirror, that's fine
if it's the drivers side you need to get it replaced ASAP as it's illegal to drive your car without it.