If half of the miners quit, the blocks would be produced every 20m. That's still fine, the network works, albeit the mempool will start congesting.

If 5/6ths of the miners quit, the blocks will be 1h long. It's becoming a serious usability problem, but the network still works. However, fees will dramatically pick up (assuming demand for blockspace is still there), so it becomes more profitable and some of the miners might plug back in.

This is BTW why you should keep some inbound and outbound liquidity in Lightning, so you're not affected for everyday usage.

If there is a massive capitulation, say 99% of miners are gone for whatever apocalyptic reason (meaning, they will not rejoin for economic incentives), there's always the nuclear option of hard forking a single off-cycle difficulty adjustment.