You stay calm by having options. And by staying calm it's easier to think of more options. And that's your job when you're running a lean start up because you're always just about to run out of money. Brainstorm to get your creative juices flowing to think of where you can get more money. There are some ideas on this thread already (sell products, collect on your A/R, lay off some of your team, ...) and there are plenty more if you can take a quiet, calm moment to think about more options.
I'd be happy to brainstorm with you if you want to set up a call.