Multisites working nicely until this morning, now I am unable to access the admin area for either the master or slave sites

http:www.mastersite.com/administrator/
and
http:www.mastersite.com/administrator/

both give me the message;
"No configuration file found and no installation code available. Exiting..."

The config files exist for both domains and as far as I'm aware, nothing has changed that might produce an error.

Would greatly appreciate your input ASAP as this is a site that is due to go live today!

This message is reported by Joomla when the configuration file does not exists or is empty and when the installation directory is not present.
You must check if your configuration.php file is present and check if the size if greater than 10 characters.
When the configuration file is not present and when the installation directory is present, this should display the "joomla installation" - here in this case, the joomla installation is not present.