What are the failure mode(s) for flying cars in case of power loss?
As I am closely working on some of these projects, I think I can answer.

Let me start from:


  if the power fails


this is a rather extreme situation, since everyone is including emergency functionality in case the batteries get below a charge guard level, triggering an emergency landing (still powered).

So to have the "power fails" scenario that your phrasing suggests, you would need a sudden fire or failure of all the battery packs installed on the craft.

What's more likely is that one of the engines will fail, but that's less critical, since all these crafts will have 8 engines as a minimum exactly for this scenario. And once again, this will trigger a powered emergency landing.

Nevertheless, you are right in assuming that these crafts can't autorotate, and some projects are evaluating/have evaluated the inclusion of emergency parachutes similar to the ones seen on some general aviation fixed wing aircraft.