Website updated to 4.6 – Now can't log into Website

Internal server errors are usually described in more detail in the server error log. If you have access to your server error log, generate the error again, note the date and time, then immediately check your server error log for any errors that occurred during that time period. If you don’t have access to your server error log, ask your hosting provider to look for you.

You can also try renaming the plugin folder. Using the file manager provided by your host rename wp-content/plugins to wp-content-old