Why am I shedding so much hair? Will it grow back?

My hair has been shedding ridiculously for the past year and it hasn't stopped, it's so bad I'm beginning to see my scalp and my hair line is becoming less dense. A traumatic incident happened around about a year ago and I went into depression/crash diet/drugs & alcohol for an extensive amount of time till eventually I quit it all and turned my life around. Since then I've focused on my health tremendously however I have noticed my hair has not gone back to it original state. What is wrong?