If you used darktable 3.8 it should have upgraded the databases and config files. Those upgrades are not backwards compatible, darktable 3.6 won’t understand them and will fail to launch.
You need to go back to your last 3.6 databases. There should be backups named something like ‘before upgrade to 3.8’ for both library.db and data.db in the config folder, copy them on top of the current ones and launch darktable 3.6.
Caveat: you will lose anything you’ve done on 3.8. And if you saved edits to xmp files, those xmp will become our of sync with the database. If you are asked about this, you need to overwrite the xmp with the database version, as 3.6 won’t understand them anyways.