Since restarting the whole database is not always an option, there is a better way:
1) check which service exactly is not running (e.g., in SAP HANA Studio, right click the system --> Configuration and Monitoring --> Open Administration; there in the 'Landscape' tab you are usually able to see which services are not running)
2) start missing services (starting them from the 'Landscape' tab usually does not work, so you could do it from console of your server):
- go to the folder of your instance, e.g.
/usr/sap/XDB/XDB01/
su - <xdb>adm
(where <xdb>adm
is the admin user of your instance)
- start missing service, e.g.
exe/hdbnameserver &