If you have a plan to get out of this, tell staff the company is struggling, here is the plan, please work very hard to make it work.
If you don't have a clue what to do, tell them the company is winding down, and to expect to be made redundant in whatever timeframe. Make sure you can pay them fully up until whatever timeframe it is!
Well, that's a crappy situation.
Normally when you get to this stage, its time to think about turning the lights off and exiting the company gracefully. Are you arranging a sale of all inventory/office furniture/releasing leases? If not, you should be.
You should also be doing all you can to raise funds (PE/VC maybe?) - but given that you're already in the "oops the lights are out" you might have left that too late.
What you tell staff now largely depends on what you've worked out as the way forward.
If you're just turning all the lights off, then let them know, let them know when their last day is, offer them help to get new jobs and make sure they get all their pay. They were never going to be billionaires if your company took off - they didn't sign up for risk, you did. So they should have their payments met ahead of you (as a presumable risk taker, and of course you are ahead of your boss).
If you have a maverick plan to set things right (or if this is a cyclical situation ~ and is somehow the first time you were aware of this- ok it's not cyclical, is it?) then you need to first start working on selling the company or IPOing or whatever.
And then once you have a plan on what to do when it all implodes, you work out your maverick plan, provide benchmarks against when the maverick plan is failing and so on.
If you do have the maverick plan, then tell your staff that your staff is floundering, and you really really need this maverick plan to work, and ask them to work their tails off for it. They probably will - nothing drives people together like a crisis.
But you owe it to yourself and everyone in the company to work out how to gracefully shut down and pay everyone their wages. Make sure you have that plan.