Traffic stops flowing via fortinet vdom

I am running 5.4.1 on fortinet VM64 with couple of vdoms.

Checked memory is around 60% and cpu is normal.

the traffic stopped flowing via interfaces as graph dropped down.

I couldn't ping default gateway or google ip.

After sometime firewall VM got rebooted and traffic starts flowing.

On the hyper-v it says rebooted by guest operating system.

This has happened twice now over 1 month and customer is not happy as I cant explain it.