In her first grass-court final, Andreescu will play Caroline Garcia on Saturday.
"I am sorry that I had to withdraw today before my semifinal match," Halep wrote on Instagram. "But unfortunately I woke up this morning with a blocked neck and this is not allowing me to perform to the best of my ability."
Andreescu's last title was her breakout U.S. Open victory in 2019, when she beat Serena Williams. Her last final was in April 2021 against Ash Barty in Miami, where she retired with an ankle injury.
Andreescu, who took time off to recharge and work on her mental health, missed the Australian Open and returned to the tour in April.
While the Canadian was able to rest up and prepare for the final, Garcia had to spend close to three hours on court to beat fellow French player Alizé Cornet 7-6 (9), 3-6, 7-5.
Garcia saved match point at 5-4 down in the deciding set before winning the next three games as Cornet struggled with an apparent right leg injury which restricted her movement.
Andreescu and Garcia have not met before.