Multisite networkpage is out of reach

I just upgraded two plugins and now I can not get on the dashboard of my network. I get the following error message:

Fatal error: Call to undefined function wp_get_current_user() in /home/jufhelpt.nl/public_html/mariellevansleen.nl/wp-includes/capabilities.php on line 1281

Who knows what I should do? Because I can not even uninstall the plugin, because I can not reach the network page.

I have a multisite (http://www.mariellevansleen.nl)

Thanks for your help 🙂

Marielle