So, I've managed to get Windows 10 and Ubuntu 14.04 LTS dual booting -- I have grub set to come up for a few seconds on boot so I don't have to mash F12 when I want to boot to Linux (windows is the default). It's been working perfectly for a few months.
Just now I tried to boot to Linux, and Grub helpfully didn't come up. I tried a few times, no such luck. I checked that it was actually booting to Grub, not windows boot manager by mashing F12 on boot and choosing Grub (the default). That didn't work either.
I haven't done anything that ought to affect anything. Ubuntu 16.04 LTS, installed from USB
UPDATE / 2017-12-15 10:55
I've borrowed a linux-on-a-stick drive from a friend, so I can run from it and edit Grub's config files. I could delete the secondary partion (from windows' partition manager) and reinstall, but that that takes forever so if anyone has a simpler solution that would be great.