MacBook Pro mid-2012 laptop Mojave HPFS partition on HDD crashed.

My HDD is divided into 3 partitions: (1) Mojave, (2) Sierra, and (3) Mavericks. I primarily work in the Mojave volume, and it has updated itself to HPFS at some point since upgrading. This morning when I opened the laptop, QuickTime Player was open so I hit CTRL/Q to close it. The whole thing came crashing down: a Restart loop began that always ended with a gray screen and multiple-language lines saying the computer had restarted because of a problem. I finally interrupted the loop by holding down the Option key to get a list of available bootable volumes. Mojave wasn't on that list, so I restarted in Sierra. Then went to Disk Utility (which I'd just been looking at last night while discovering and learning about the whole APFS system), and when I clicked on the line for Mojave it said, "AppleAPFSMedia Unitialized."

So what do I do now to get Mojave back online, and save as much data as possible?

I have Time Machine backups of the Mojave volume from at least 2 days ago, maybe even from last night though I can't be sure until I can get back into that volume. The MacBook Pro.sparsebundle file says it was modified at 8:35am today. That's about the same time that I woke up and discovered I'd left Quicktime Player open, hit CTRL/Q, and started the whole crashing mess. But 2 days ago is good enough for backups - I don't use it as a work computer, there's nothing critical there, and I made new clones of the Sierra and Mavericks partitions with Carbon Copy Cloner last night. (Didn't clone the Mojave partition since the Mojave volume clone on my external hard drive was in the old format, and the one on the MacBook Pro HDD was in HPFS format, so I needed to research what to do before replacing the old clone.)