When she awoke, her daughter Christina, then 17, was cradling her head in her lap, her cheekbone was broken and her eyelid was cut from a fall.
"I had no idea what had happened," recalls the editor-in-chief of the Huffington Post. Adds Christina, "It was terrifying."
Even scarier? Huffington had no idea what was wrong with her; she spent weeks undergoing tests. As it turned out, the answer was simple: She was exhausted from working 18-hour days, seven days a week while expanding the Huffington Post, which she co-founded, into the hugely successful news site it is today.
Looking back now at her 2007 collapse, Huffington tells PEOPLE, "I'm so grateful for this wake-up call that changed my life."
Now the 63-year-old reformed workaholic – who still maintains a packed schedule but tries to get seven to eight hours of sleep each night – is urging others to realize that rest is part of success, in a new bestselling book called Thrive: The Third Metric to Redefining Success and Creating a Life of Well-Being, Wisdom and Wonder.
For much more on Arianna Huffington's life and new book, check out this week's issue of PEOPLE, on newsstands Friday