Hello Nicolas.
Are you using the older IBM Case Manager (ICM) (eg v5x) or the current BAW Case? (Hopefully the latter!)
Normally when you add a new property to a case type, the deployed case type class object is updated to include the new property. Existing instances that use that class will get the new property, but their value will be null.
When you open a case details page where you have exposed the new property, it will show blank for those previous instances.
If you page is not loading, perhaps you have some customizations (custom widgets, scripts, etc) that are failing because of your change. Check in the browser's debugger for errors when you try to opening the page.
Also note that while BAW Case still supports the classic widget pages from ICM, the Client-side Human Service UI framework is the recommended way to build highly granular, dynamic UIs for both your process and case solutions.