UPDATE 24/01/14 Whole of Google+ Down – Database backed up to 21 January 2014.
The Reason Why Google+ was down
Apps Status Dashboard
Google+ Errors just before big announcement
Big Google+ announcement coming soon and Google+ starts giving an error Oops… the system encountered a problem (#500) – Retrying in 1s…
Update 16:31 (UK Time):
+Vic Gundotra has just posted
“Just lost half the power in the building/block.
We are furiously working to bring in a generator for the equipment that drives the live feed. We will likely be delayed by 15 minutes. Sorry about that folks!”
source: https://plus.google.com/+VicGundotra/posts/hcNs9WPiTqU (for those who can get Google+ working)