ZFS modules suddenly stopped loading - Debian 10
It sounds like you updated the kernel:
- to a newer one that isn't ABI compatible with the old one so weak-modules couldn't be used
OR
- you are using ZFS kmod package and ZFS kmod for your new kernel wasn't available
OR
- you are using DKMS ZFS modules and they for whatever reason didn't build automatically when you installed the kernel, or at least the initramfs wasn't rebuilt after they were built.
You will need to boot with your old kernel again and investigate further.
If that is not an option e.g. because you deleted all the older kernels, boot a Ubuntu livecd and mount our file systems from that for troubleshooting, since Ubuntu comes with ZFS support as standard.