3. Claw your way back.
Don’t necessarily set your sights on completely winning back your client's business . It may make more sense to incrementally edge your way back by taking on smaller pieces of business.
With this "foot in the door" tactic, try to score a smaller "yes," a one-off project. Or you might even provide an entirely different product or service than you have offered before. Then set out to turn a smaller yes into a bigger one.