Since updating my iPad 2 to IOS6 and activating my icloud for backup, my iPad has permanently locked. Message on screen is "iPad Not Backed Up". I click ok but no response. Cannot turn power off and cannot restore via my pc itunes. What are my options?

Thanks to everyone who responded and related subject helped. Holding the home button and power button simultaneously for 10-15 seconds does unlock the screen. I can now use my ipad again. However, a futher problem has arisen since updating to IOS6 and activating icloud at same time, Although not sure if its related. The problem is that when connected to my PC the itunes message appears 'Cannot read ipad'. It instructs you to restore but keeps timing out after about 1 hour. This maybe internet issue but can't understand why itunes cannot read my ipad. May need to completely erase all content & reset. Or any other suggestions why this has happened?