[Samsung Galaxy S7 Questions and Answers] crazy_dr0id:
Not sure why you don't just restore a nandroid backup in full or in part? (wipe cache, dalvik, system then restore system, wipe caches again before rebooting system...?)

My guess is that you probably were not on oreo/8.0 unless you somehow took ota while rooted... I use the Canadian variant but it's pretty much is the same process every android device whether using odin or fastboot to flash

recovery being used is twrp 3.1.1.0? deselected enable digest verification in twrp settings? have you done a backup before today, if so did you swipe right to write to system? I'm more a supersu girl so if not flashing supersu zip to root after touching system and/or kernal you will be stuck at samsung screen of frustration indefinitely due to dm-verity verification fail which can be fixed by flashing system partition of nandroid before booting (if previously boot was working), flashing supersu to update stock kernel, or flashing a different kernel which i'm not totally ok recommending to you at this point in time given that I still don't understand how you don't know if you were on oreo, nougat, kitkat, jellybean, MM, or gingerbread >.<

your best bet if you absolutely have no idea what you are doing is to backup data/complete nandroid while waiting for full firmware download from samsung to finish, might as well go with oreo but may have bugs so might want to wait a bit still and take March or April release date, wait a few hrs so do backups then mount to usb and transfer to pc (not mac right? using samsung usb if possible in 2.0 port) then read up on guides, all in ones, get rooting things needed bc you'll need to re-root. heck reformat your microsd too that sometimes helps (if bored when waiting for firmware to complete downloading - data easily corrupts and I have no idea what led up to this event or has been happening since soooo yeah, might be worth a try)

when download is done using whichever odinis applicable to match firmware you are flashing, turn off antivirus, run odin as admin, connect phone using usb after putting into download mode (btw just to verify dev option - usb debug and oem unlock were selected before starting all of this right? assuming yes sinice rooted already w/ recovery but heads up, will need to do this again after), put extracted ap file in ap, don't reboot only f reset time checked in options in odin, when that's done, reboot phone back to download mode flash recovery/twrp, then go from there and eventually restore data using something like titanium backup if you had that already going, if not... this is why we do backups in recovery before playing with things (esp if unsure what you have or where you are going and how to get there) or regularly so if things go wrong, you have semi-recent image to restore to so loss is minimal.

good luck!

ps. i use whatever firmware matches my carrier (or used to match carrier if unlocked) but here we can use almost any so go with ones w/ least amt of bloat.

pps when set up take a nandroid and save it somewhere like onedrive also you can install custom rom but always like to have clean starting place when i've screwed up and been stuck in similar place before

at least you can get to recovery, hopefully you have backup to can restore from easily! :)