As hibernate.conf doesn't seem to work I did this :
echo 'case "${1}" in
hibernate|suspend)
sudo service openvpn restart
;;
esac' | sudo tee /etc/pm/sleep.d/20_restart-openvpn
sudo chmod +x /etc/pm/sleep.d/20_restart-openvpn
This create a script called "20_restart-openvpn" (you can change the name of the script) and make it executable. I hope this can help you too ...