After 5.18.4 Upgrade, Cases are Missing from the List
The issue here was missing XML files. If you run an upgrade and accidentally override the CiviCase XML configuration files, then this will happen.

The solution is to put back those files and CiviCRM may pick them up and import them into the database. If not, you may need to manually copy and paste the XML definitions into the civicrm_case_type table.