Without diagnostic trouble codes my best guess would be the speed sensor on the transmission is in some way at fault, disconnected or wire broken. The inability to detect output speed on the transmission would prevent, in turn, the ABS from functioning. The ABS module needs to accurately compare speed sensor inputs from transmission and wheel hubs.
Answers & Comments
Without diagnostic trouble codes my best guess would be the speed sensor on the transmission is in some way at fault, disconnected or wire broken. The inability to detect output speed on the transmission would prevent, in turn, the ABS from functioning. The ABS module needs to accurately compare speed sensor inputs from transmission and wheel hubs.