I have a Gentoo system installed as a VB guest. I boot it every Sunday to run my weekly "emerge --sync && emerge -uD world && emerge --depclean". I do this because although I am not using Gentoo as my main OS atm, I don't want to lose the Gentoo way of thinking.

I booted it up yesterday and during the bootup process, I saw one red * among a sea of green. Something about it couldn't mount /home. I actually had to reboot the guest a couple of times to figure out what it said. I spent last evening poking around Grub sites convinced that there was a way that I could get in through the grub prompt. This ultimately failed. I did learn through my searching that I could use the Gentoo LiveCD image stored on my hard drive to access the broken system. What file in /var/log should I be looking at the determine my next step? Unfortunately, I did not find anything in /var/log/messages. I've run xfs_repair and the drive will mount when I order it from the command line, but as you know, during bootup one doesn't have access to a command line. What other questions should I be asking that I have not asked previously in this thread? I'm kinda a Gentoo noob. I used it successfully from 2005 to 2015, but I didn't have to do much with it; it just worked. I guess I trusted my luck too much.

A few random suggestions. Caveat: I don’t know anything about Gentoo idiosyncracies.

Have a second (or first?) look at /etc/fstab to detect anything suspect in the /home line.
Check if the /home disk/LUN/partition is available at the tome of mounitng.
Create a user whose $HOME is not under /home, log in as this user and snoop around.
If your system uses systemd, check the journal for relevant messages (journalctl command).

I also wonder: Can you mount /home from the live DVD without repairing it first? If not: Could there be a problem in the way this machine is stopped, perhaps causing corruptions in the filesystem?

I don't think the problem is in /etc/fstab, as I don't think it would have mounted when I said "mount /home". It did. This system does not use systemd, so that's not it.

I was just thinking about what I was doing the last time I was successfully logged in. I was rebuilding the kernel! I bet THAT'S causing my current issue! Thank you for your excellent advice!

Rebuilding the kernel repaired the issue.