Macbook freezes during start up

I Have a MacBook Air, it may have auto updated to the Sierra last night. She worked beautifully yesterday, today she wont boot up. She freezes during the startup, the taskbar gets to the half way point and the images shift to the right and down then everything stops. Anyone else have this problem? I could use some help! I literally was powering up so I could back up my files and now I cant do anything... Any help would be greatly appreciated!