1. Check that YugaByte DB is running
First, ensure that the expected YugaByte DB processes are running on the current node. At a minimum, the tserver process needs to be running to be able to connect to this node with a CQL client or application.
Additionally, depending on the setup, you might expect a master process to also be running on this node. Follow the instructions on the check processes page.
2. Check cluster level issues
Next, check the list of cluster issues and the respective fixes for each of them.
3. Check logs
Inspect the YugaByte logs for more details on your issue. See this page for more details on where to find and how to understand the YugaByte log files.