In some WordPress configurations, after upgrading from CiviCRM 5.11, the CiviCRM navigation menu bar has disappeared.
The problem only happens with certain configuration situations and seems to be related to changes in the asset caching system.
As a quick workaround, you can disable Asset Caching at:
I was battling with this for some time now. Try changing the PHP version. For me 7.1 worked on IIS