Complete internal IT meltdown it seems, and many people not able to unlock (or lock) their booked cars.
https://twitter.com/search?q=zipcar&f=live
Seems it is worldwide as reports from both UK and US in there.
Their app isn't showing active reservations, which prevents you using the app from locking/unlocking it.
My reservation started 1h30m ago and I haven't been able to get into the car. That's my weekend plans hosed and a wasted hotel booking that I doubt I'll be able to get a refund on.
CS rep just said that "engineering were on it" and no timescale for a fix.