Your configuration files are automatically backed up to the system dataset once a day. As long as you didn't have your system dataset on your boot drive (it is moved to the pool by default), there's a good chance you can get a recent version back.
Look at post 35 in this thread
https://www.truenas.com/community/threads/update-went-wrong.74084/page-2#post-514298. It gives the location where the files are placed by default and describes how to get to them.
You'll see a section where Chris describes a "kind of tricky" procedure to get to the files - I recommend that you use Midnight Commander - a DOS-like file manager built into FreeNAS/TrueNAS - type "mc" on the command line to get to it:
View attachment 45018
I pulled it up there in the shell to show you, but it's better to SSH in to your box and use it that way if you have SSH set up:
View attachment 45019
You can see my config file from today at the bottom of the list.
I'm pretty sure that it's this process that Dan T. was referring to.