1. Is the page published?
A page with no publish date exists and can be edited, and visitors cannot see it. This is the cause most of the time, so check it first: the Published switch on the page's row in the list.
2. Is every page above it published?
A published page under an unpublished parent cannot be reached, and neither can anything else beneath that parent. Publishing the parent can bring a whole section back at once without touching any of its pages.
So check every page up to the home page, not just the page itself. If a section and all its pages answer "not found" together, suspect the section. The same applies to books: a book page shows only if its unit and its book are published too.
3. Is the publish date in the future?
A publish date in the future keeps the page hidden until that moment, even though it is set. A page can also have been taken down by an unpublish date, which clears its publish date when it passes.
4. Is the address the one you think?
A page's address is its parents' slugs followed by its own. Renaming a page does not change its slug, so the address can differ from the title. If the page has a custom URL, the site links to that address, while its own address keeps working too (see Custom URLs and redirects). A rewrite rule can also send the address elsewhere before the page is looked up.
5. Check it the way a visitor would
Signed in to the admin, you are shown unpublished pages: the setting "Show unpublished" is on for each admin user unless they switch it off. Your own browser is therefore the least reliable place to test. Open the page in a private window, or ask someone who is not signed in.
Still not found?
If the page and every page above it are published, the dates are in the past and the address is right, pass it to whoever maintains the site's templates, with the exact address you tried and what you saw.