Fatal error: Call to undefined function

I’ve been able to post to my blog in the past without problem. I haven’t added a post to my blog https://www.questcareer.com/wordpress/
in quite a while, but I’d like to now. I haven’t made any changes that I know of, but I’m getting this error message when I select the Write a Post link from my blog’s WordPress dashboard:

Fatal error: Call to undefined function: wp_get_current_user() in /home2/ann/questcareer-www/wordpress/wp-includes/functions.php on line 339

Can you help me with this?

Thanks,

Ann