The java agents aren't separate processes, they are modifications to the code of the application/jvm so they run with the application. This means that the only way to restart the agent is to restart the application. This is the only option to restore instrumentation if the agents could not reach the collector at startup.
It might be a good idea to inspect the agent logs to determine the exact reason that it was not able to instrument the JVM last startup. Perhaps your collector is not located in an appropriate location for this agent? Let us know what messages exist in the log.