Do you have a backup of pwenc_secret in your /data/ directory? (Usually named pwenc_secret.bak.)
ls -l /data | grep pwenc
Do you have a history of your auto-generated config files?
ls -l /var/db/system/configs-*/
If you’re lucky, you might be able to combine either pwenc_secret or pwenc_secret.bak with a config file from around the time you remember last accessing (successfully) your encrypted dataset(s).
The combination would be a simple tar file that contains both files. (If you use pwenc_secret.bak, you’ll need to rename it to pwenc_secret before creating the tarball.)