The major version upgrade attempt failed and the subsequent revert left your Postgres service running on a new, empty volume while your original data (~8.9 GB in the postgres-volume) is sitting detached but fully intact on our storage. Your data is not lost, and it can be recovered by reattaching that original volume to the service.

The main thing is why is the upgrading failing?

Every upgrade attempt failed at the same preflight check: your PG 16 database references a loadable library (extension) that is not included in the target version's image. The check logged "Checking for presence of required libraries ... fatal" and aborted before any data migration, which is why the process appeared to hang. The same blocker hit both the version 18 and version 17 targets. To identify the problematic extension, connect to your current PG 16 database and run SELECT * FROM pg_extension;, then cross-check each extension against what the target major ships. Drop or replace any that are not available in the new image, and retry the upgrade. Your original data (~8.9 GB) is still intact on a detached volume and has not been lost.