Help- My blog is gone after trying to install a plug-in

I was trying to install a plug-in called “wordpress-seo” and it said I need to update my WordPress in order to install but I wanted to take the time to go through backing up my WP files before upgrading so I just closed my browser and made a note to do that later. Well now I’m trying to get to my blog and after logging in I got the message:

Fatal error: Call to undefined function get_user_meta() in /home/content/87/4218187/html/blog/wp-content/plugins/wordpress-seo/admin/class-metabox.php on line 747

And now, its not even taking me to the log-in page and only saying:

Error establishing a database connection

I’m not a programmer or website expert by any means, so I’m really lost here. Any help would be greatly appreciated.
Thanks so much!
Nicole