Ok, worked it out. I use WordPress Secure, that is supposed to record your ip address in an .htaccess file in wp-admin. Then access to wp-admin is denied to anyone with a different ip address.
I had a look at this and the ip address isn’t mine. I changed it and now I have wp-admin again.
The big question though is why the ip address changed.
I wonder if someone has been able to get in and do something malicious. If so where do I start looking? I can’t see anything different looking at the site or at wp-admin.
What do I look for to see if I’ve been hacked?
Thanks