Create a script that contains the following (you could name it something like pingtest.sh):
if /bin/ping -c 3 <ip>
then /sbin/service network restart
Be sure to put your actual IP in the place of <ip> above.
Make the script executable and put it somewhere secure such as /root/scripts.
Modify root's cron to call the script only at reboot:
Of course you could add more to the script such as logging or an "else" for what you'd like it to do if the ping fails. You can also add logging to the cron entry.