Today I cannot enter my website and I have seen that the static ip has been changed. How is this possible?
Probably you have misconfigured the network in your VM due to leaving the defaults settings in the VM, that means that your ip is ephemeral. As comment above says, you have to reserve a static external IP address.
You can learn more information about how ip addresses work in Google Cloud platform in the following link.
I suggest to reserve a external static ip address, login into the VM through ssh and update the IP address in the webserver.