Unfamiliar

I am having a case of "who are you" with someone I know...knew...Basically a friend of mine seems to be acting really different and every time I talk to them I keep saying to myself "who are you" towards that person. I don't know how to handle this because I feel that this person and I used to be really close, but it's like I can't even relate to them anymore. I don't know how to handle having this person as just another acquaintance rather than a really close friend. It's the weirdest thing ever, but that's what it's turned into! I don't even know this person anymore. I've seen this happen to other people, where they change so much that you don't even know them, but it means more when it's this person. Change can be good, but I'm not sure that this change is good, at least not for our friendship. Quite frankly, this person is someone that I want to keep being friends with because I'm hoping they'll go back to being their old self. That's not something I should expect though. I feel somewhat obligated also, because how could a friendship so awesome and amazing just be thrown away??? I realize that maybe I've changed too, but I think it's mostly this other person. This...is...weird...I have run out of words to describe what I'm feeling right now...