The problem has been identified and searches are now working normally again..

Post-mortem analysis

Logs in the Google Cloud backend indicated that the service unavailability was caused by the Elasticsearch master node becoming completely unavailable as a result of the hard disk capacity of the hot-content tier server overflowing. This was most likely due to a bug in the script I use to bulk import topics into Elasticsearch. After contacting support, they were able to temporarily upscale the hot-content server to a larger capacity within 20 minutes. I am now going through the data and cleaning up any duplicates and other data that might have overflowed the server.

This highlights the importance of using managed resources for production services. If I was running the server myself, it would have probably taken days to recover from this.