Toni Ko founded the cosmetics company NYX in Los Angeles in 1999. She set her targets on a gap in the market between low-end drugstore cosmetics and pricey brands sold in department stores. NYX took off during the advent of social media--and during the recession. She executed its sale to L'Oréal, for an estimated $400 million, in July 2014. But the sale of her life's work left her feeling completely untethered.

--As told to Christine Lagorio-Chafkin.

I started the company when I was 26. By the time I'd sold it, I was 41. I had lived my entire life single, focused, and working. I thought: "I'm going to sell the company and have  work-life balance!" And I would travel. I thought, "I'm going to be able to let go. I'm going to retire. I'm going to get a margarita, sit on the beach, and read a book."

Selling the company took three years of preparation. It's kind of a plateau--and then there is a drop-off point. The drop-off point is the day after you sell the company.

Words cannot describe that moment. I felt like a balloon that had been filled with oxygen to its fullest, and someone came and gave it a small stab with a needle, and it just went pop. And there was nothing. Blank. Black. Dark. Hole.

I initially thought I would go and drink champagne and celebrate. But what I did the day I got the wire transfer in, and the transaction confirmed and completed, was pack my stuff from my office and walk quietly out. I went and fell asleep for, I think. 14 hours straight.

The next morning, I was a little bit confused. My life for 15 years had been waking up in the morning and getting ready and going to work. But I opened my eyes that day and realized I had no place to go and no reason to wake up. I just went back to sleep. That went on for several days. Soon, I was  bored out of my mind. I didn't know what to do with myself. How many days can you spend on the beach? How many times can you go shopping? My life became very redundant. It was meaningless. I felt like I didn't have an identity. I felt like I wasn't adding value to my life or to society. I just felt like a loser, almost.

Eventually, I started an investment company, and then I started to build a real estate portfolio. I was never not doing something at any given moment. But nothing was very exciting or meaningful to me. I'm a products person--I need to be in an environment where I create products. That's when I have the most joy, fulfillment, excitement--everything.

Literally three days later, I started dreaming up other businesses. Eventually, I settled on sunglasses and, within months, launched a company called Perverse. It made me feel whole again. It gave me a reason to wake up in the morning.