Today I logged into Windows on my dual-boot Ubuntu 16.04 / Windows 10 laptop, which I don't do very often, and Windows installed a heap of updates.
Seemed very fine to me until I restarted and tried to start my faithful Ubuntu - and to my dismay got stranded in a primitive text-only screen with a few lines of text that ended in:
So, as had happened before, Windows again borked my boot loader. Sigh. (And some profanities which I won't repeat here).
I already had repaired my boot loader in similar circumstances before, using Boot-Repair, but couldn't find my Boot-Repair Live USB so decided to boot into a Ubuntu 16.04 live USB and start Boot-Repair from there (following the "2nd option: install Boot-Repair in Ubuntu" on the Boot-Repair wiki page).
I did "Recommended repair" in Boot-Repair, but at the end I got a message that it had not worked due to "Locked-ESP detected", and I was recommended to create a /boot/EFI partition at the start of the disc, using eg. gParted.
I launched gParted and saw that there was already a /boot/efi partition on my hard drive, but I also saw little key icons next to the names of all the partitions - which rung a bell with the "Locked-ESP" message.
So I right-clicked the partitions one by one in gParted and selected "unmount", which made the key icons disappear.
I then started boot-repair again, with the Recommended Repair option, and lo and behold, it fixed my system neatly.
I do feel that it should be explained on the Boot-Repair wiki page that the hard drive has to be unmounted before attempting to run Boot-Repair with the 2nd option (installing it in Ubuntu"), but for this one edit to the wiki I can't find the courage to go through the process of getting the right account and permissions on the Wiki. So hopefully someone that already has those, will do the job...