Hi Mark,
I recent update required an update to emoncms settings.php file. This file is located in /var/www/emoncms/settings.php, the update script looks to see if there are any user changes to the settings file and if not the settings.php can safely be updated with default.emonpi.settings.php.
What I think has happened in your case is easier you had made a customisation to settings.php or your are running an old version of emonSD.
This fix this, you will need to connect via SSH and manually update the settings file (creating a backup first):
$ rpi-rw
$ cp /var/www/emoncms/settings.php /var/www/emoncms/backup.settings.php
$ cp /var/www/emoncms/default.emonpi.settings.php /var/www/emoncms/settings.php
$ rpi-ro
Emoncms should now load up. I would be interested to see your emonpiupdate.log logfile to work out exactly what has happened during the update. This can be downloaded from the Setup > Admin > Download log. Please could you attach the logfile to this email.
If emoncms is still not running you will need to copy and paste emonpi update log file manually via ssh this can be done using
$ cat ~/data/emonpiupdate.log
Then copy the text on display