The 22 people on the ride were safely evacuated around 8:30 p.m. after being stuck for nearly three hours. Two of the riders were "transported to a local hospital for precautionary measures," Magic Mountain spokeswoman Sue Carpenter said.
Brandy Chambers told KTLA-TV she was on a nearby ride when she "saw the Ninja go into the trees."
The Ninja will remain closed while inspectors survey the track and the area, Carpenter said.
"The safety of our guests and employees is our No. 1 priority," she said.
Magic Mountain bills the Ninja ride, introduced in 1988, as "The Black Belt of Roller Coasters," racing along a twisting track at 55 mph.