[Update-manager] Ubuntu 10.04 won't boot after kernel update (Sep. 23,2010)

Hold shift at boot and try booting an older kernel (if you have one). Does it work ok?

I cleaned the old one after it restarted OK the first time around with the new krnl. :( I'm on a 16Gb SSD drive, so space is an issue. Shift only brings me to recovery - and it stalls somewhere before loading X and I still can't figure out where.

16Gb is plenty. Keeping the older kernel for at least week is a good policy as you will have a fallback, each kernel also only uses 120Mb space.

If you boot now, what is onscreen. I suggest you remove the boot options to show the splash, so you can watch the boot happen.

Yes, I know, Im trusting QA testing so if it reboots OK I get rid of the old kernel (any tutorials to restore an older kernel onto a dead drive?). It's a good policy to keep at least a week and I'll enforce from now on. If I boot now it's what I said before - stuck prompt after fsck. Log file says it tried to load spamassassin after that. Then nothing.

I would be surprised spamassassin is the problem. Problems after update-manager install a new kernel are related to kernel itself.
Unfortunately, I'm not currently on a Ubuntu computer, so I can't provide you a verified solution.
1) Boot on Ubuntu Live CD
2) System->Administration->Synaptic
2.1) In search window, enter vmlinuz
2.2) Install the kernel you are interested
2.3) Do the same for initrd (not sure of name)
3) Check you have in /boot the needed files for previous kernel
4) Mount hard drive and copy these files in <mounting point>/boot
5) https://help.ubuntu.com/community/Grub2#Reinstalling%20from%20LiveCD
6) Check new file <mounting point>/boot/grub/grub.cfg
7) Reboot while holding shift key
8) Select previous kernel in Grub2 menu
9) If it works, remove latest kernel and run "sudo update-grub"
10) Reboot to check

@actionparsnip: How can we prevent update-manager to automatically install new kernel ?
@DanD: don't hesitate to post to ask explanation. As I said, I have not tested this process.

What is the status of your bug ? If it is solve, please mark the answer which solves the issue, or else provide your solution. If not, I will have some time this week to help.

Sorry to not have followed up sooner; ran into USB issues unrelated to this problem. I will be able to test the steps you provided tonight (I am on New York time). One thing is a bit unclear in step 5, specifically manually editing grub.cfg to add the now (old rather) kernel. I might have to do a kernel install twice and then diff the file to see the right places to change in the cfg on the mounted drive.

5) You are not supposed to edit file grub.cfg.
Commands to type are:
   sudo mount /dev/sda1 /mnt
   sudo grub-install --root-directory=/mnt/ /dev/sda