why isnt my emergency brake working at all?

What is your question?

Last week a neighbor was attempting to back up and gunned the back of his pick up truck into the front of my car. the e-brake was on and my car was in 1st gear...the car got pushed so hard it backed up over a large curb the underside of the car scrapped along the curb...one of my rear tires was on the grass one over the curb...i tried to park my car in my driveway and all of the sudden it rolled backwards...the brake was on/car in 1st gear/wheels were cut. although the damage to the front of the car does not look too bad im very confused as to why my ebrake wouldnt work. no problems with it previous to this ontop of that it started 1x after that and it wont start now...i just spent $530+ for a new alternator so i was having no problems with it at all. that it until the ebrake stopped braking.