Since we were talking about backstabbing friends a couple of days ago, I've decided to talk about what to do if a friend starts randomly ignoring you. I suppose the first thing to do is go confront them and ask them "dafuq's going on?" I'm sorry, I haven't really thought this through...
First thing to do; Consider this; is this friend actually ignoring you? Are you just being over dramatic or are you over analyzing it? If you think you're just being silly, maybe tell your friend how you're feeling and plan to spend some time together.
Second thing to do; Figure out if it's only one friend who's ignoring you or a group.
If it is just one person, talk to them when no one else is around. If your friend is ignoring you because they've made a new friend, it's not really fair for you to intervene. Yes, it may be extremely annoying (trust me I've been there) but maybe you should give this new person a chance? You might have something in common.
If it's a group of friends, pick out the one you know the best or the one who may feel guiltiest about ditching you. Get them to tell you what's going on. Don't get angry or this will get back to the rest of the group and it'll make you look bad.
Third thing to do; Ask them what's wrong. Maybe you've upset them? Maybe they've made a new friend? Maybe you've made a new friend and they feel left out? Maybe you've outgrown each other? Whatever the case may be, don't be afraid to talk and let your feeling out.
Note; Do not cry or beg them to be your friend. That'll make you look like a weirdo.
That's really all you can do. I'm afraid. It is hard to lose a friend, but I think the "there's plenty more fish in the sea" rule applies here. You just have to put yourself out there. If I can maintain a few healthy relationships, there's hope for anyone!