My greatest fear as a startup founder isn't to fail, it is to become a zombie startup. Kind of like in the 6th Sense when Bruce Willis doesn't realize he is dead and tries to have a nice dinner with his wife, there are startups out there who are still "operating" but might as well not be.
The first thing you need to do is acknowledge the reality of your situation. From there, figuring out what to do next is a lot harder and a very personal and contextual decision, but you should embrace it with vigor. Don't waste single moment of your life, or the time of those on your team, to begin plotting the next step.
Danielle also points out that if your startup has failed, hard, don't shy away from calling it a failure and moving on.