Magento 2 admin is not working

If admin is not opening either after migration or upgration or you have change the server then you have to check few things first.

select * from core_config_data where path like '%custom%';

This Command will give you all the option from db related to admin settings. Pay attention to these 

• -admin/url/use_custom
• - admin/url/custom
• - admin/url/use_custom_path
• - admin/url/custom_path

From etc.env file you can check 

return array (
  'backend' => 
  array (
    'frontName' => 'admin',
  ),

Also check from db that ssl is enable for admin or not.