All static routes were missing in customer's Fortigate 300D firewall

Our customer's Fortigate 300D firewall went down on 21 June 2021 Monday at about 5 PM Singapore Time. When I went onsite to our customer's office, I managed to bring up and login to the firewall. I discovered that all the static routes were missing. My colleague imported/restored the latest firewall backup configuration file and restored almost all the static routes. The next morning our customer had issues connecting to VPN and could not connect to a certain wireless network SSID as well. I discovered that a default static route was missing. After adding the default static route in the firewall, all the problems were solved.