Open your wp-config.php file through your webhosting control panel. Scroll down until you get to the line that describes the multisite as shown below:
/* Multisite */
Change the word 'true' to false
When you do the above, you 'll be able to access your theme and plugin, disable all of them.
Go back to your cPanel and reverse the change you have made i.e change the word 'false' to true
Go back to your wordpress dashboard, you should be able to access your network multisite admin area.
Last work: Activate your plugin one after the other and see which was actually giving the problem. you may delete the plugin permanently or better still if the plugin update is available, update the plugin.