Moved to the DLP sub-forum for better handling.
Try logging into the appliance via ssh and issue "service stingray restart".
There's a recent version that had this issue with auth, so you may also want to make sure you're running the most current version.
Try adding the discover to manager and then try login.
Try rebooting the NDLP Discover..........
I also faced the same problem but in my case I am able to take ssh but not able to login & my device was showing as critical in System option.
Once I rebooted the problem was resolved...........
We have already restarted the server once. But still there is problem.
Try to restart the below mentioned services:-
service stingray restart
service jboss restart
service mysql restart
If still it doesnt work then u have to take help of support team...........
What version of NDLP Discover are you running ?
Was it a managed / unmanaged device?
This error sometimes occur in pre 9.3 version very often due to mysql issue.
Best to check if your stingray daemon is running or not first after reboot.