Can't Sign Into iMessage

Since upgrading to ios7 on my iPad4 I cannot sign in to imessage (or facetime either, not that I use it!) I keep getting the following message: "imessage activation could not sign in please check your network connection and try again" I have done everything short of doing a factory reset. I've done some searching here and followed links/instructions in other posts relating to imessage problems but nothing helps. What can I do?? (Everything works fine on my iPhone 4S)