Need Help. Installed beautiful OMV 5 home network server /plex server & bios update broke boot loader?

I apologize for I am a technical person - but more of a hands on engineer - than a coder.

I used techno dad life's guide (thanks! can I contribute?) - a couple weeks ago
"Openmediavault 5 (OMV5) Stable Complete Install and Setup including Portainer"

to install OMV 5 onto a repurposed dell PC with a 32 gig ssd and a shucked 8TB WD HD for storage inside on two sata ports (just disconnected the sata to the third dual boot win7/win10 drive - which can be accessed manually by changing back to ahci in bios plugging in the sata to that drive and booting into that) but I installed OMV 5 perfectly from an SD card - with the bootable installer on there. Using all the tools that I really didn't understand -

steps i did-
downloaded openmediavault amd64 iso 5.05
downloaded and installed putty
balenaetcher - installed etcher
burned open media vault iso to 16gb sd card pulled sd card (in plugable sd card usb device)
set pc bios to boot from usb devices
went through the steps to install onto the 32gb internal ssd
installed and restarted and followed the guides to login as root, etc.
used techno dad's configuration/settings guidance - to setup OMV.
Then used portainer and docker and plugins etc. and got the plex server on there
OMV - with my shared folders - was visible and very quickly accessible to all my home network computers- mac and windows - streamed some videos, shared files etc. great.

But - I updated the bios on this old dell machine as I wanted to upgrade the cpu and ram - some cheap ebay upgrades available - and after successfully off line flashing/updating the bios - I restared to reboot the OMV server - and nothing....

I get "reboot and select proper boot device". I had disabled ahci, have legacy enabled, found the 32gb ssd with OMV on it in the bios, etc.

I am afraid I do not understand how to get into the debian to fix the bootloader - that the bios update messed up. I then put the SD card OMV installer back in and tried to find a rescue/fix and just didnt' know what I was doing when I got into the CLI.

I am very proficient following technical instructions- building/upgrading/fixing computer compenents, solving problems - but I just do not know my way around unix/linux/debian/command line at all - and frankly don't know how to get to it to see what is wrong. In searching for anyone with a similar problem I found a few threads but frankly I can't exactly follow how to get in and execute commandes to fix whatever my problem is.

I am postive my successful OMV/Plex server is there waiting to be "awoken" unless I have messed it up by running the installer again (i aborted before it let me pick a place to install to/partition - since I'd previously installed to the 32 gb ssd perfectly fine. So I don't think I wrote over it. I'd rather get it back and running rather than go all the way through and reinstall/load/configure everything.

Really was loving the OMV- was so sick of the terrible external drive on netgear router usb port home storage solution.

Thanks so much for any explicit step by step help.

How can I rescue this successful OMV 5 installation?

small update-

have booted from the SD card installer and manged to get to a Debian Installer Main Menu and one of the options is
"install the Grub boot loader on a hard disk".

could this be what I want? I think my bootloader from my original install was Grub(though I really don't know) - and could I somehow fix/reinstall my grub bootloader without wrecking my successful OMV/Plex setup that was ll configured and working well before.

How can I "look at" what I've got on my 32gb installation/boot disk and fix whatever the bios change screwed up.

Also tried "execute a shell" and got into 'busybox' and the/bin directory but I don't know how to see what's there (i have dos commands in my head from 80's engineering school lol).

Sorry - I am a total novice at this as far as terminal/command line/debian/grub - but capable of solving these problems with some explicit guidance - and then will know exactly what I've got and how to look at it/check it for proper function to get me back to my OMV home network file/plex server. Thanks in advance.