Fucking shit. So I turned off the computer yesterday. Today it got stuck right after decryption. The hardware for my server onto which I intended among other things to do more regular and complete backups will arrive tomorrow. I think some of the more important data is backed up on other computers, but some data that has only sentimental value isn't. Well, I hope a live boot will still be able to fix this. The timing at least makes for a funny story if nothing else...
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Assuming that something has gone wrong with how you have things set up- an install of a new kernel package/generation of the kernel image, that didn't go quite right, etc, rather than a hardware failure, there should be options.
Assuming you're using Grub like a sensible person who doesn't jack off to the beauty of the UEFI design, simply taking the option of using a slightly older kernel in your grub menu might work.
If not, you should be able to boot into a live environment, and then either use a GUI file manager to mount the volume, or, if that doesn't work, you should be able to open it using the 'cryptsetup open DEVICEPATH MAPPEDNAME' then 'mount /dev/mapper/MAPPEDNAME /mnt' or whatever.
EDIT: If everything mounts up fine,
once you've backed up the data, if you can figure out what the issue is you should (now being booted up with a running kernel), be able to chroot to the root install and fiddle fuck around to get the right /dev and other ancilary filesystems set up. At which point you can then work 'normally' (rerunning grub install scripts etc etc or whatever is neccessary to unfuck things). Doing that in whatever way your particular arch setup is set up for of course..
EDIT 2: I see Arch comes with a couple of nifty scripts for easing the cockpain of chrooting when you've fucked up your install. I wonder why that is! Pretty handy anyway. Guess I'll be installing the debian package for arch-chroot
just in case.