From wikipedia:

"Zhang's Ph.D. work was on the Jacobian conjecture. After graduation, Zhang had a hard time finding an academic position. In a 2013 interview with Nautilus magazine, Zhang said he did not get a job after graduation. "During that period it was difficult to find a job in academics. That was a job market problem. Also, my advisor did not write me letters of recommendation." The reason behind this is that Zhang's research pointed out the mistakes made by his advisor Tzuong-Tsieng Moh's previous work. Moh was very unhappy with this and refused to write the job recommendation letter for Zhang. Zhang made this claim again in George Csicsery’s documentary film Counting From Infinity while discussing his difficulties at Purdue and in the years that followed. Tzuong-Tsieng Moh, his Ph.D. advisor at Purdue, said that Zhang never came back to him requesting recommendation letters.[8] In a detailed profile published in The New Yorker magazine in February 2015, Alec Wilkinson wrote Zhang "parted unhappily" with Moh, and that Zhang 'left Purdue without Moh’s support, and, having published no papers, was unable to find an academic job'"

His advisor wrote his own version of events here: https://www.math.purdue.edu/~ttm/ZhangYt.pdf

Crazy how this one guy derailed a genius like Zhang. Glad to see he found his way back, but sad that he had to endure almost a decade in the wilderness - at one point even homeless (living out of his car) - working minimum wage jobs just to get by.