Why did my compute instance lose it's public IP address?

Summary:

Until a few hours ago, my Free Tier instance had a public IP address which was assigned when I created the instance many many months ago. It became unreachable this morning and when I look at my instance now it says:

Instance access

The instance requires a public IP address to connect to it from the internet. You can assign either an ephemeral public IP or a reserved public IP to a private IP for this purpose.

Public IP address: -

Why did my instance lose it's public IP address?