calDAV - ipad locked

[Declaration]

Hi, I tried to log into my ipad, but it came up with a blue box saying "Enter the password for the calDAV account "Yahoo"". I have googled this, and it was suggested that if I entered my talk21 e-mail address password, then it would clear. However, although I have done this and pressed 'OK', nothing happens. Worse still, I cannot access my ipad, as I cannot get beyond this screen. Has anyone else had this problem? I should say, I am unable to switch it off, because it won't let me slide the bar across the screen