Were the characters that changed very dramatic? For example, your character was blue with green stripes and is now pink with red spots? (Basically a completely new unknown character.) Or was it minor, for example they almost seemed texture-less, or as if something was missing from them? I just am trying to figure out if your game just isn't "applying" character textures correctly, however if there's new "friends" added to the account, I'd look at the possibility below.
It is most likely your account if the characters have changed completely, AND there's new usernames on your friends list - it's a possibility someone has gained access to your account. I'd highly recommend changing your password, and changing anything you deem important in your profile here on the site. Make sure you never give passwords or any vital information that someone could use to gain access to your account.
You could even try making a new account here if push comes to shove. The registration opens daily at random times for approximately two hours. If you refresh your page every hour or so, I'm sure you'll catch it soon. Good luck, Jan.
Edit: I just thought - Your maps were gone? You can't delete maps unless whoever got into your account had access to your computer. Was it a sibling, family member, or friend by any chance?
