take a needle (those things that your wife or mother uses to repair your socks !), don't use it to torture the suspected guilty but carefully 'pin' the bent diaphgram and work your way to reshape it using the needle as a lever.
After that, with a SMALL drop of sylicon glue close all the neddle holes in the cone (a toothstick is a great tool to place the glue)
PS: give the glue the right time to dry before crank up the volume again and (seriously now) check if the coil is not damaged : gently and slowly push and pull the cone with your fingers (apply the force near the center) and listen: if you hear a kind of scratch noise, it could be that the coil was ovalized by the hit and is touching his 'walls'.
Good luck
sandro