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My LUKS-encrypted Ubuntu 17.10 install normally would boot to a display that requests a passcode to decrypt the drive and then would boot to the Ubuntu login display. Instead it is now booting just to a black display. I have tried booting it to GRUB by pressing Right Shift but it then just displays GRUB loading. and freezes. I have tried the Alt+←, Alt+→ trick mentioned here, which is listed under the heading "LUKS encryption", to try to get a terminal LUKS dialog for the decryption passcode but this doesn't do anything, and the black display remains, so the solution presented there doesn't work. I can boot to a live USB Ubuntu distribution but I don't know what to do from that.

How can I approach this problem further?

EDIT: After a number of attempts at rebooting (which changing nothing manually) they system eventually booted successfully. I have no idea why increasing the number of boot attempts or letting time pass should be able to fix a problem like this, and I'd like to understand this. However, following this, I encountered the problem again (after Ubuntu froze) and again am seeing the black screen on boot.

While the booting was working this time, I followed the suggestion of David Foerster and managed to record a Boot-Info log from when the boot was succeeding and then subsequently managed to record a Boot-Info log from when the boot was just going to the black screen.

I have compared these logs and nothing jumps out at me but perhaps a more trained eye will see something that I don't.

BlandCorporation
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    It looks like LUKS is only mentioned in one answer on the question I linked. So maybe it's not helpful. Then again, maybe this has nothing to do with LUKS. If you feel LUKS is the problem, you might want to edit the title to clarify. – wjandrea Apr 24 '18 at 01:39
  • @wjandrea Thanks. Yes, I actually referenced that question in my question, saying that I tried its solution, which doesn't work. Is there some way to get the terminal prompt of LUKS to appear or to force GRUB to load successfully? – BlandCorporation Apr 24 '18 at 22:52
  • @DavidFoerster I added the heading ("LUKS encryption") under which the steps I followed are listed, and I already had those steps described in this question, basically trying to get LUKS to display its dialog in a terminal instead of in a GUI. As mentioned, this didn't work. There is literally only a black display, no terminal output. – BlandCorporation Apr 25 '18 at 14:43
  • @DavidFoerster I have already included these details. I had them in the question originally. As I said, I did Alt+←, Alt+→ as is suggested in the LUKS encryption section of the solution, again which I already referenced. I don't know how to be clearer. – BlandCorporation Apr 25 '18 at 15:59
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    Alright. Looks like the Grub installation may be broken or misconfigured. Could you please run Boot-Info and [edit] your question to include a link to its resulting info log? I don't know if it can repair encrypted installations but it'll include enough info for me to describe how to do it manually. Thanks. – David Foerster Apr 25 '18 at 16:02
  • @DavidFoerster Thank you for your suggestion. Somehow the system briefly started booting successfully and now is booting to black screen again. Happily I managed to get Boot-Info logs for a successful boot and an unsuccessful boot. I've edited my question with links to the logs. Could you by any chance take a look? – BlandCorporation May 10 '18 at 00:07
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    The Boot-Info output looks fine and the Boot-Info program itself thinks so too. It doesn't hurt to reinstall Grub in case we missed something. Can you try that please? Since I don't know how Boot-Repair handles encrypted partitions, I recommend manual methods like in Scott’s answer. To decrypt LUKS partitions you need to use sudo cryptsetup open <NAME> <DEVICE> --type LUKS in step 2 and on success the decrypted partition will appear at /dev/mapper/<NAME> to use in step 3. (You can choose <NAME> freely.) – David Foerster May 10 '18 at 09:20

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