Prohibited sign - APFS internal drive is not mounted. I have a data backup and just need to get my iMac to work again. How do I get a clean start?

Hi all. I came home from work to find my less-than-two-year-old iMac screen black with a white prohibited circle. Maybe there was an automatic update while I was out that didn't go well? I tried turning the computer off, letting it sit for 5 minutes, then turned it back on...same black screen with white symbol. That exhausted my personal computer knowledge, so I turned to Google. I tried rebooting in safe mode...same black screen with white symbol. I tried restarting with the NVRAM thing (Command-Option-P-R) and it cycled on and off a few times with the chime, then went right back to the prohibited sign. I learned how to get into Disk Utility.

In Disk Utility under "Show All Devices," I see two major categories: Internal and Disk Images. Everything under Disk Images checks out fine with First Aid. Under Internal, the ST2000DM001 Media shows a status of Uninitialized, but checks out fine with First Aid...no problems. Under APPLE SSD SM0128L Media, the APFS Physical Store disk0s2 is greyed out; it says it's not mounted. Clicking the Mount button does nothing. Running First Aid does not fix it. I'm considering trying the Erase or Restore options, but my computer knowledge is limited, so I'm hesitant to start clicking without knowing all the steps after that. I did try to reinstall the operating system, but no drive shows up to reinstall it onto. I am not especially computer-smart, so I've never tried anything advanced on this computer...it should all be pretty factory standard.

All of my personal data is backed up with Carbonite, and it's reassuring to see those files online on another computer. So I'm not worried about personal data in this attempt to fix my iMac...a clean wipe back to factory settings is fine with me.

Is the missing internal APFS disk the likely source of my problem? Any ideas on wise next steps?

Thanks!