As you know, Nova works in concert with other OpenStack services, including Keystone, Glance, Neutron, and others. As a result, it is possible that problems with these services may manifest themselves as problems with Nova.
The Nova database
Nova makes use of a database to support its functionalities. If this database is not running or is improperly configured, it can cause your Nova installation to be unusable. To troubleshoot the Nova database, begin by making sure that the database engine is running. For example, when using MySQL, you can check the status by running the following command:
service mysql status
If the database is running, it will be indicated in the output of this command:
mysql start/running, process 15628 ...