My Hadoop is not working, what can I do ?

Keep calm and ;-)

• First check your logs
• Is the service is running ? (netstat -nat | grep ...)
• Is it possible to access it ? (telnet ip port)
• Is there a problem linked with path, java libraries, environment variable or exec ?
• Am I using the correct user ? 
• What is the security system in place ?
• Are nodes well synchronized ?
• What about memory issue ? (swap should be desactivated also)