Hi Ripley, don't feel lonely, you can always come here and chat to us! :-)
There are many reasons why your friend may have "gone under-ground." I think the important thing here is for you to be the strong friend - maybe send a message just to say you want to make sure she is ok, if she could just text or email to let you know, that would be great. Maybe everything is just fine in her life, and she doesn't realise how much you depend on her, so this will be a not-too-heavy way of checking in. But maybe she has some things of her own to deal with, and if she suspects you need her for emotional support as well, she may just feel she can't be that strong right now. Sure, she should have told you, but maybe she doesn't want to hurt you. The important thing is not to make her feel guilty for being busy or having other friends - this will make you seem needy to her, and while we all want to help our friends in need, we also want to laugh and have fun with them sometimes too! :-)
I really think you are right that this should spur you on to make more friends, instead of thinking you are no good at this, use this example of a ten year long friendship as proof you can do it well! I am sure you can make more friends if you set your heart to it, it just requires being a little brave, and being prepared to follow up with people you hardly know - in the hopes you will get to know them eventually! When you meet a person who seems nice, get their email or phone number - and do call, txt or email them. It seems hard at first, but it gets easier. Txts are a good way to start! Once you have a selection of friends, you won't rely so heavily on this one person for your happiness, and also, you may even find it will difficult at times to schedule them all in!
Good luck Ripley, and remember, you already have friends here!