You might have noticed that your Ububtu server does not respond to ping requests after a reboot. In Ubuntu the networking is not started by default on boot. This might be causing the issue.
To start networking on boot, you may follow the below given instructions.
1. create a script in /etc/init.d with the following contents.
#!/bin/bash service networking start
2.set the file to executable mod
chmod +x <script>
3.update your runlevel configuration.
update-rc.d <script> defaults
Now try to ping it from a different ip to verify.