I guess the term for this in banking world is called a "bank run". The bank only has 10% or so of the money actually available at any given time. So 90% of customers would be completed f****d in this scenario trying to withdraw their money. But what happens to crypto in this case?
You are misrepresenting the number on the exchange is an imaginary number or can be printed as they wish. Regulated centralized cryptocurrency exchange and a bank are different. The most likely scenario in cryptocurrency is when the exchange itself is a scam, thus they run with all of the customer cryptocurrencies.
But let's say, I follow your hypothetical scenario, if there are no buy orders, then simply the bitcoin that everyone had will not move a.k.a not being sold, thus they can't have any FIAT. And the bitcoin is frozen because there isn't any activity. Let's say even if in one exchange the selling pressure is way bigger than the buy, well the market will reach its own equilibrium because there is an arbitrage opportunity.