lose (one's) footing
1. To stumble and/or fall, typically during a physical activity such as walking. I sprained my ankle when I lost my footing on a hike. Be careful not to lose your footing while you're on the ladder!
2. By extension, to lose one's stability by entering a precarious or unsettling situation. I loved my job, so I really lost my footing when I was laid off. I'm just worried that he'll lose his footing if he drops out of school now—there's no guarantee he'll ever go back.