Check the AEM error.log file and see if you are getting the "AuthenticationSupport service missing. Cannot authenticate request" error trace.
If yes, then try to re-index the complete repository by deleting the "index" folder under the path /crx-quickstart/repository. And then restart the instance and monitor the logs.
If you still get this error, try to restore the instance from the last healthy backup.
This type of issue comes up when the instance is shut down abruptly and the repository could not be shut down gracefully.