Hortonworks HDP , heartbeat lost in one of the 3 nodes

Well the provided information is not sufficient, anyway i will try to tell you the normal approach I take to debug this.

1. First check if all the ambari-agents are running, use the command ambari-agent status.
2. Check the logs of both ambari-agent and ambari-server. Normally the logs are available at /var/log/ambari-agent and /var/log/ambari-server. Logs should tell you the exact reason for heartbeat lost.
3. Most common reasons for the agent failure would be Connection issues between the machines, version mismatch or corrupt database entry.

I think log files should help you.