(Possible short-cut for experts reading this: Boot-Repair result is http://paste.ubuntu.com/5776162 . Thanks!)
I'd been using 12.04 LTS on this machine since November, and had a stable system. I'd clicked on the software updates each time they were offered, but the updates Sunday resulted in linux being unbootable: it got partway through the boot-up, then froze. Windows still worked from the Grub menu. I dropped into a root shell and successfully backed up the files that I hadn't backed up earlier. I burned a new 12.04 LTS disc to try to reinstall, but the disc didn't boot. Rather than messing with that further I decided to try 13.04. This disc worked fine for the installation.
The laptop has two hard disks. The Ubuntu system is on /dev/sdb6, and Win8 is on /dev/sda3. It originally had Win7, and I foolishly "upgraded" to Win8 before doing the 12.04 install. During the 13.04 install I told it to put Grub on /dev/sdb6 rather than accepting the default, since that's where the linux partition is -- I'm guessing this was a mistake.
After 13.04 installed (apparently successfully), on booting I got
error - file not found
grub rescue >
I couldn't find anything useful to type to the "grub rescue" prompt. 'Help' doesn't work, for example.
After some googling I booted Live from the 13.04 install disc, mounted /dev/sdb6 (the linux partition), and did "sudo grub-install --root-directory=/mnt /dev/sdb" and "sudo update-grub". More fails, then ran Boot-Repair, with result in http://paste.ubuntu.com/5776162 .
After more googling I found that I needed to change the disk boot order in the BIOS (probably I should have done this much earlier in the process), and then I got the Grub menu on booting.
From the Grub menu, Ubuntu boots into a running 13.04 system that still has my data files. It appears to be intact enough for me to tune it back to where I need it... assuming I can run CUDA on 13.04.
However, Windows doesn't boot from the Grub menu. Here are the options:
Ubuntu [boots into 13.04]
Advanced options for Ubuntu
[list of old versions]
Windows UEFI bkpbootmgfw.efi [ error: no such device D00A-07CD error file EFI /Boot/bkpbootx64.efi ]
Windows Boot UEFI loader [ `/EFI/Microsoft/Boot/bkpbootmgfw.efi' not found - Press any key to continue ]
Windows 7 (loader) (on /dev/sda3) [ invalid EFI file path ]
Windows Recovery Environment (loader) (on /dev/sda5) [ invalid EFI file path ]
Windows 8 (loader) (on /dev/sdb1) [error: can't find command `drivemap' error: invalid EFI file path ]
The BIOS gives four options for booting from disk:
Windows Boot Mgr - goes to the Grub menu
P2: MatshitaBD-CMB error: file `/boot/grub/i386-pc/normal.mod' not found
P1: ST9750420AS: same as the P2 error
ubuntu (P0: ST9750...) - goes to Grub menu
Again, the Boot-Repair link is http://paste.ubuntu.com/5776162
Thanks in advance!