Scared and worried about depressed friend?

alright so I have this friend, and for the last week or so her depression has been really bad. she's been like this for a few months but before she would be fine and it would just hit for a day every 2 months or so. now Im really scared and worried abut her. she doesnt talk to me the same way anymore like she used to and she's just become so anti-social. She goes to sleep really early and she's just lost interest in everything! Im really afraid that she's going to hurt herself or something or worse and it scares me to the point where I cant sleep, cant eat... cant get her off my mind.. and I feel like this is all my fault. I mean i told her some things the other day (no i wasnt being mean or anything, just talking to her) and then all of a sudden THIS happens! the worst part is we dont live in the same state so I cant go and just spend time with her and help... I feel so useless.. I just dont know what to do anymore, I feel so guilty. if anyone knows how I can possibly help her I'd appreciate it so much! I feel like Im losing my best friend in the world..