Advice please...

Advice--for a friend...please help....

My best friend in the whole world has stopped talking to me like she use to and now she is hanging out with all the wrong people...how do I help her...I have tried so many times but she won't listen to me anymore...We still are good friends just not like it has been...((We've been friends since we were babies))

So do I just let her go and forget about it and hope she gets through life ok ..... or keep trying to help? And if I keep trying to help her...what should I do?