The installed system was 12.04 LTS, and the Muon Update screen said an upgrade to 14.04 was available. I clicked Upgrade after making sure 12.04 was up to date.

Several hours later (it's faster to reinstall from scratch!) I clicked on "remove obsolete packages", and a bit later rebooted the system as directed.

All I got was
Is there a simple way to recover?

I successfully used the Boot-Rescue-Disk and can boot 14.04 now, but I get pop-up windows telling me that Pressing the button to send the report appears to do nothing, and I cannot find anything in the logfiles to indicate what the problem might be.

This is not something the average user would want to see or deal with.

Try booting to the system and;
[code]
apt-get install grub-pc grub-common
[code]
on the assumption that grub was simply screwed in the version upgrade.

Clean installs are not only faster but a lot more reliable. There is no reason to loose data if you are installed on more than one partition. Tell the installer not to format the /home. Remove your ~/.foo files for reconfiguring your system later but letting the installer set up those files as there may be some changes in how they are organized.

Backup is, of coarse, highly recommended.

Did you remove all 3rd party sources from your sources.list? Did you run the "ppa-purge" command before the version upgrade if you are using any ppas? The presence of either will cause some real problems in version upgrades.

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Since grub was the problem, I could not boot the system. I could have booted from a live distro and used chroot, but used the Boot-Rescue-Disk which worked and is much easier.

After I clicked the "Upgrade" button I was told the sources had nothing appropriate and asked if I wanted to replace them (not sure of the wording). I clicked yes.

I did not run ppa-purge. I did not know about it. I think the only ppa was for googleearth.

Today I re-installed from scratch in a separate partition and all is well.