Messenger hasn't worked like it used to for several years now.
Do what @Glennlab says if you want to send one.

A cur and paste from my previous community senate post

One more time, in order to use the message function.
1.Delete all the messages between the two of you
2. Go to the person's profile page.
3. click on the message function there.
4. Type your message and send.

So far this is the only work around that works. If you are going to continue to communicate I suggest you exchange emails, or throw away emails, or phone numbers.

OH, DEAR! Isn't this the way it began the last time it disappeared?

Nope.