Hi there!
There can be several issues regarding your problem:
1. Do you use wordpress.org or wordpress.com to built your website?
2. If you have multisite go to the network admin and there will be Plugin tab
3. Try to access http://www.yourdomain.com/wp-admin/plugins.php (replacing http://www.yourdomain.com with your site’s domain name).
If it’s accessible, then explore the theme and plugins that might have disabled the menu item.