There are a number of things you can do. Here are some of them:
1) run a search for that host, and start with the last 15 minutes. Increase the time to see how long it has been off line.
If it populates with current data, then it automatically came back online. Sometimes forwarders will go offline when the system reboots, if it takes to long, but they come back by themselves. The missing forwarder message will go away in about 15 minutes.
2) Log onto the server with the down forwarder and check the status of the forwarder: service splunk status.
If the forwarder status is stopped, then restart it with service splunk restart.
If the forwarder fails to start, post the error message here.
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