Having enough liquidity in a business is always important, but in a crisis it becomes fundamental. If you are managing or advising a business with depleted revenues you have to have a proper understanding of the cash position, both now and as it is likely to evolve. @davidtilston sets out some basic truths - get a rolling cashflow forecast up and running asap, and keep it updated. By doing so you will be able to see problems as they develop, and stand a better chance of navigating through them.
I have dealt with a number of liquidity situations in the past, ranging from the severe to the potentially terminal. I pass on one useful lesson I have learnt. Get a 13 week rolling cashflow forecast started as soon as possible, ideally this week, and update it weekly.