Well, that's what happened to me - my archive drive failed, meaning I couldn't access anything from a few months ago back to 'way back when' including all originals, masters and derivatives - I was screwed in other words.
That drive simply vanished from my computer, and in LR it was greyed out.
I think the answer partly depends on what kind of backup routine you have - software you use and access you have to that data.
The backup software I use (on Mac - called SuperDuper) creates an exact copy of what I want, in this case an exact replica of my archive hd, which is fully accessible like 'normal' data, without any messing about. And luckily I do it regularly.
In LR I imported another folder, namely my backup, and let it build previews overnight. Basic import settings, i.e. in it's current location and with no presets or other settings applied on import. I can carry on where I left off, as I needed access to some of those pics. No problems at all, everything was preserved from the backup (labels, metadata, snapshots etc).
So now my backup is live, and when I get the repaired drive back [alternatively purchase a new drive] I'll copy the backup to the original drive, sync the folders in LR of the original archive folder (remove the backup folder if I choose to), and continue working from there, and revert my backup back to it's dutiful 'backup' status. Of course I now have a backup of my backup in the interim
[To close - the drive had a known firmware issue, it's been flashed, and is on it's way back to me. Of course, the data is of little significance now as it's changed in the mean time though it's good to know they were able to restore it - esp where the originals are concerned]
