I fail to see how that would ever happen. Authorized participants [0] are always in the market for ETFs. If an ETF share price is crashing out of line with the index it tracks, they will step in and buy shares, swap them with the ETF issuer for the shares of the underlying stock in the index, and sell those shares for an arbitrage profit.
Even if one of the underlying stocks becomes illiquid, a big enough price divergence on all of the other liquid stocks would make it profitable to eat the loss or hold the illiquid ones (risky, but remember, there are many authorized participants competing with each other so if there is some way to make an easy arbitrage profit, they will find a way). You'd basically need the entire market to become illiquid.
Even if one of the underlying stocks becomes illiquid, a big enough price divergence on all of the other liquid stocks would make it profitable to eat the loss or hold the illiquid ones (risky, but remember, there are many authorized participants competing with each other so if there is some way to make an easy arbitrage profit, they will find a way). You'd basically need the entire market to become illiquid.
[0]: https://www.investopedia.com/terms/a/authorizedparticipant.a...