How do I get rid of iMessage "inviting" to sign in?

[question everything]

I was out of the country for a week and texting was useful.

I use my iPhone for texting but everything also appears on my iPad which I had with me.

My last text did not go because I was not connected to a WiFi network. Was OK. We were landing.

But now on my iPad I am locked from my texts by a large screen telling me to Sign in with my Apple ID to activate iMessage.

There is nothing I can do. I don't know what iMessage is, I don't care, I just want this message to disappear.

Any suggestions?

Thanks.