"Sorry, you are not allowed to access this page"

Your website is fine and visitors don't see this: it only appears in the admin. It comes on a plain page with nothing else on it.

The roles, in one line each

Administrator

Everything on the site, including plugins, settings and users.

Editor

Publishes and manages everyone's posts and pages.

Author

Publishes and manages their own posts.

Contributor

Writes their own posts but can't publish them.

Subscriber

Only their own profile.

If it's one page

1. Did you get there from a bookmark, an old email or a link inside a plugin? If that plugin has been switched off or deleted, its pages are gone, and WordPress shows this message for them. Go to Dashboard and look for the page in the left-hand menu instead.
2. Is your role too small for it? Look at your left-hand menu. If you see Profile where an Administrator sees Users, and there's no Plugins or Settings, you're not an Administrator. Only Administrators can open those.
3. Need it? Ask an Administrator on your site to change your role (Users › All Users, Edit under your name, Role, then Update User), or to do that one job for you.

PageKiwi for WordPress is one example: its page and its "Check this page" button are for Administrators only, so an Editor who follows a link to it sees this message. That's on purpose.

If it's every admin page, even for Administrators

If nobody can get into the admin, something changed for everyone at once, and it needs your web person the same day: until then, nobody can update the site.

• After a move to a new host or a new database. WordPress stores each person's role in the database under names that start with the site's database prefix (wp_ unless someone changed it). If the prefix in the wp-config.php file and those names no longer match, every account loses its role.
• After installing or changing a plugin that edits roles or locks down the admin (role editors, membership and some security plugins). Plugins can add or take away what each role may do.

Don't try to fix the database yourself. Send your web person the message below.

If your site is part of a network

Some organizations run many WordPress sites together as one "network" (WordPress calls it Multisite). If you're not a member of the site you opened, you get a different message: You attempted to access the "[site name]" dashboard, but you do not currently have privileges on this site. If you believe you should be able to access the "[site name]" dashboard, please contact your network administrator. Ask the person who runs the network to add you to that site.