I just encountered this issue. After restarting identity server, the following error appeared in the logs:
System.Exception: Error processing block: Sitecore.Plugin.IdentityServer.ConfigureSitecore ---> System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.AggregateException: Certificate cannot be obtained with defined parameters.
This lead me to investigate the thumbprint in the sitecore.IdentityServer.host.xml file. When I went to see if that SSL cert was still valid, I realized that it no longer existed. Someone must have removed it so I updated the thumbprint to a valid SSL cert, restarted the identity server and I was back up and running.
Note: The above error only showed AFTER restarting the identity server.