If its just one page, you can do any of:
- Use permissions to hide it
- Work on a new version without publishing it
- Copy the page, hide the copy with permissions, work on the copy, then swap the copy in once done.

If it is the whole site, you can do any of:
- go to the dashboard and put the site in maintenance mode
- Make a clone of the site, work on the clone, test the clone, then swap the clone for the original