This just happened one day. I usually have auto-log in on, but at some point, it asked me to write in my password. I didn't think much of it at first, but every time I clicked to "sign in" it would fail to sign in for a second or two and then ask me to sign in again. I am getting no error message, and I can still go in as a guest. But all my bookmarks and previous sessions are lost.
I tried to use the "forgot password" function, and it said it successfully changed it, but when I went on to try and log in with the new password, it just gave me the same result as above.