Can’t post or log out

My site was working fine last night, but today I get this message whenever I try to publish a post:

“You do not have permission to do that.”

I get similar messages when I try to deactivate a plugin, change themes, or log out. I’ve uploaded the latest wp-admin and wp-includes folders, with no effect. My host says they’ve changed nothing. The site itself is running fine; I’ve just lost the ability to administer it.

What can I do? Thanks.