Macbook updated and deleted everything?

My Macbook Air updated by itself while it was plugged in and closed last night and it deleted EVERYTHING all my files are gone, the background picture is different, it doesn't even ask for my password anymore when I log in. What happened to all my stuff and how do I get it back? Even weirder when I plugged my phone in to try to back it up it says it cannot be backup because there is not enough space on my computer and must delete some files... yet there is nothing on my computer...? Please help me!!