Sometimes on admin widgets page , the drag and drop function or widgets not load properly. When go to click help or screen options, those will not open either. It seems like a scripting issue on just the widgets page, as all other pages in the admin seem to work fine.
1.Check IF there is plugin or Theme Confict
The easiest way to do this is to install “Health check ” Plugin.Troubleshooting allows you to have a vanilla WordPress session, where all plugins are disabled, and a default theme is used, but only for your user.In troubleshoot mode user visitors will not effect.Once you install the plugn you will see an addional option on plugin page.Click on troubleshoot to check plugin one by one which is causing the issue.
2. Replace wp-admin Folder
If above method doesn’t fix the issue, try reuploading the wp-admin folder.Goto your site cpanel, delete wp-admin folder and upload fresh files.If you don’t have cpanel upload it via ftp.