Fatal error: Call to undefined function

I’m trying to access my admin panel, but when I log in I get the following error

Fatal error: Call to undefined function pib_render_button() in /home/queenofs/domains/melissamiksch.com/wp-content/plugins/pinterest-pin-it-button/class-pinterest-pin-it-button.php on line 137

My website is working fine from a user standpoint, but I can’t get in to the back end to edit anything. I’m fairly certain this is an issue with my pinterest plugin…. but I can’t deactivate it if I can’t get in.

Here’s what I’ve tried: manual update of 4.2.1 via FTP. Using both Chrome and IE9.

website is https://www.melissamiksch.com