The template for this display is not available.

i finished and launched the site. It worked for a week and now I get this error when visiting the site. I took the site down because my client saw this. How do I work around this?

Word 'error' don't give us many details. Could you write something more ? Maybe paste error message here ?

"The template for this display is not available. Please contact site administrator". This is the error I received.

Please switch template to 'Beez' and check whether even Joomla! is working.

I think the problem was that I used the same database for two different domains. When I changed one, it changed the other. I just did the site all over again.

You are very brave person if you use the same tables in two separate installations. Database configuration you can check in configuration.php file in root of your installation directory.