Wordpress CiviCRM site unusable after database upgrade from 5.4.0 to 5.5.1
People using 4.7 are out of date - 4.7 is end-of-life.  You should definitely go with 5.x for most use cases.

I would check your error logs in the ConfigAndLog directory - then come back here and edit your question to add the relevant logs.  In general, there's a troubleshooting page in the documentation, but most of the suggestions aren't applicable to upgrade failures.