In general, when Design Studio stops working, I would check the DesignStudioBackend.properties located under **<DENODO_HOME>/conf/design-studio** folder as well as tomcat.properties in the **<DENODO_HOME>\resources\apache-tomcat\conf** folder to see if there are any misconfigurations.
Also, I would check the log files of the Virtual DataPort server and the Web Design Studio located in the **<DENODO_HOME>/logs **folder.
If in case you have [SSO](https://community.denodo.com/docs/html/browse/8.0/en//solution_manager/administration/authentication_and_authorization/authenticating_with_sso/authenticating_with_sso) enabled, make sure all the configurations related to it are enabled correctly.
Suppose if you still need help and if you are a valid support user then you can raise a support case on [Denodo Support Site](https://support.denodo.com/) so that our support team can help you
Hope this helps!