Is this a site you inherited? If so, maybe the other designer didn't upload the template, but still has it on their computer. If you made the site you should have it somewhere on your computer.
You can detach a template from a page:
I think it's the "modify" menu, then "templates", the "detach from template". I don't know how or even if you can re-attach it someway, I doubt it. So if you have a lot of pages then this wouldn't be a good idea.
The template is somewhere, you need to find it.