Resist the urge to panic
The first reaction, emergency discounts, drastic strategy changes, is rarely the right one. Give yourself a few days to really understand the reason before reacting structurally.
Understand the real reason
Ask for direct feedback, even if the answer isn't pleasant to hear. An honest reason helps you prevent the same thing with other clients; a wrong assumption can make you "fix" something that wasn't broken.
Check the real cash flow impact
Look concretely at the invoices and revenue affected, not the general feeling of loss. Sometimes the impact is smaller than it feels emotionally in the first week.
Bottom line
Losing a big client hurts, but a calculated reaction beats a fast one every time. Understand, then act, not the other way around.