The mystery of the disappearing nodes and the perils of too much coffee.
I had what can only be described as a mouse induced panic attack
yesterday.
I was adding content and moving nodes about in the Umbraco
editor with reckless abandon, when pow all of a sudden all my nodes
disappeared!

EEEK! I navigated away from the content tree into another
section in the hope that they were just shy and if I stopped
looking at them they may re-emerge. Nope, I returned to the content
section and they were still gone (Sort of the opposite of
Schrodinger's cat).
Then I had one of those rare moments of clarity and realised I
had accidentally double clicked the root content node. A quick
double click of the content node and all was well again. Just one
of those gotchas that crops up from time to time.
I put it down to an excess of coffee and over excitement at the
speed of the new Umbraco content tree.