iMessage not working

My iMessage on my ipad 3 isn't working. It used to work but when I logged out I was not allowed back on. It keeps saying my apple ID and password isn't correct. It is correct simple because I just updated some of my apps. I have rebooted the entire system and the problem is still occurring. I live in dubai but at the moment am in the UK and even there I can't get the iMessage to work...can someone please help??