Unable to View Salesforce Cases

If your organization-wide sharing default access for Cases is set to 'Private' and 'Grant Access Using Hierarchies' is enabled, only the Case owner, the owners' managers in the role hierarchy, Salesforce admins, and user profiles with the 'View All Records' object permission on Cases are able to view a Case. Case owners have Read access to the accounts that their Cases are associated with by default.
 

To check your Default Sharing Setting for the Case object perform the following:
 

In Salesforce Classic: Setup | Security Controls | Sharing Settings

In Lightning Experience: Gear Icon | Setup | Security | Sharing Settings

Check if the Default internal access is set to 'Private' and if the 'Grant Access Using Hierarchies' is checked for Case.

In addition, only Users who have been added to a Case queue will be able to access and take ownership of Cases in the queue.

Also, reference this Trailhead for additional information: Overview of Data Security.