Why might an External IP change on a Google Cloud Platform VM instance?
Instances are automatically given an IP address, which is subject to change if the server is restarted, crashes, etc. If you want it to have a fixed IP, you need to provision one.
Chances are you've lost the original IP - it'll be back into the pool of millions of IP addresses for someone else's instance to use. You'll need to change the IP in your DNS records to the new static one.
In addition to ceejayoz's answer, it should be noted that you can promote an ephemeral IP to a static IP without it changing.