It looks as if we have once again for some reason lost the entire trac.htpasswd file. I noticed this because trac refused to accept my password. I was able to reset it but then remembered what had happened before. This may have been due to the disk's being full. In any event, I've restored the file from backups (yeah!) and copied over the usernames registered since, and the one other password change that had been made.
Going forward: If anyone should at any time have password trouble, please let me know ASAP. We were lucky this time that the backup was actually made earlier today, so there wasn't much work to do. Richard PS If you had trouble reachign the website earlier, it was because I took it down so I could do the restoration.