The game logs me out and displays a box asking me to create a new player. Edit
This is commonly caused by sending either invalid commands or too many commands in a short period of time, most commonly occurring when using the keyboard short cuts provided by LOU Tweak.
You can either wait 5 minutes after getting this message and the game will automatically let you back in, continuing your current game if you haven't changed pages which has the benefit of not losing chat messages or you can go back to the home page, click the logout button at the top of screen and then log back in, you can then select your world and get back to your game immediately.
I've lost all my cities. Edit
If all your cities had castles then it is possible that they have all been captured by somebody else, if any of your cities didn't have castles then the possible reasons for this are that you've not logged in the game for too long and your account has been reset, for players ranked Earl and above this period is 30 days, for players ranked as Baron or below the period is 15 days.
If neither of the above 2 apply to you then it could be that you have accidently visited a different world to the one you originally created or that you have reset the game or that somebody you have given your email login details as done so.