BuddyPress.org

OK, I have sorted it out.

The bp-capabilities value somehow had got changed to author rather than administrator.

I edited the bp-capabilities item for user_id 1 in bp_usermeta table to be
a:1:{s:13:”administrator”;s:1:”1″;}
and logged in again and everything was fine.

If you have multisite, you may need this thread which I used to find the place to start looking for the solution.
I tried their bp_settings and it gave me a ‘you are not authorised to view this page’ message, so I used my value above for my non-multisite installation.

https://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/im-no-longer-super-admin-what-happened/

Hope that helps someone else.