As you mentioned "restart" and "google-compute-engine" tag in your question, it is possible that if IP assigned to the instance was an ephemeral external IP which means if you stop and start the VM instance or delete the instance then external ephemeral external IP assigned to that instance will be released and there is no guarantee that you will retain the same IP address back. So, IP must have changed for the webserver when you restarted the VM instance.
You will need to update the required 'A' DNS record to point to the new IP address assigned to the VM instance, if that is the case.
If planning on hosting websites, I would recommend using static external IP addresses. Static external IP address can be reserved and assign to the VM instances. This will make sure external IP address assigned to the VM instance will be persistent throughout the lifecycle of the instance.