Sudden failure to boot into any of the boot partitions

Overnight, after having worked successfully the previous day, I get this when turning on the laptop:

<code>
Checking Media presence
No Media present
Reboot and select proper Boot device
or Insert Boot Media in selected Boot device and press a key
</code>

Here's my setup:
Toshiba Satellite E45t-A4300
OS: Ubunutu 18.04

Rebooting just brings up the previous messages.

I went into the BIOS and changed Boot mode from UEFI boot to CSM boot. That worked, temporarily. I got my multi-boot selection screen, and I chose Ubuntu. Then I got this:

<code>
Welcome to emergency mode! After logging in, type "journalctl -xb" to view
system logs, "systemctl reboot" to reboot, "systemctl default" or ^D to
try again to boot into defauilt mode.
Press Enter for maintenance
(or press Control-D to continue):
</code>

At this point, it looks like it will boot into Ubuntu, but it just returns to the emergency mode screen again.

I then tried booting into Linux Mint, on the theory that the Ubuntu partition was corrupted, and that another partition would work successfully. But alas, emergency mode showed up again.

I can of course boot into Ubuntu with a boot USB, which is what I am using right now. I can operate effectively through the USB boot.

While in Ubuntu, I ran Gnome Disk Utility: All partitions checked out OK.

I am at a lost as to what to do next.