Hi Korean Visitor,
They have Loan to Value ratios that generally protect them in such events. Should the value of the crypto drop below a certain price, the collateral and loan will be automatically liquidated protecting them from any such events.
The Loan-to-Value (LTV) is what determines the amount of crypto collateral you need to provide. For example, if you want to borrow $10,000 in a collateralized loan and the LTV is 50%, then you would need to provide $20,000 worth of Bitcoin.
Most crypto lenders offer such solutions with significant crypto-based collateral protecting them from such events. It is a really good point though and something to look out for when choosing a lender. Generally, those without healthy LTVs are best to avoid as they could be problematic in a bear market.