A chartered Australian airliner began searching for the boat early on Friday, while Australian, US and French rescue authorities sent ships to an area about 3,218km southwest of Australia
Sunderland's father, Laurence, said he lost contact with his daughter during a satellite phone call and believed her boat may have rolled in treacherous conditions.
Shortly after she ended the phone call, Sunderland, who left California on her 36-foot (11-metre) sailboat in January, activated two emergency beacons.
During a blog entry written on Wednesday, Sunderland said she had spent several days in rough weather and had to patch one of her sails. Sunderland also wrote that she was having trouble with her Internet system.
Sunderland had hoped to become the youngest sailor to circumnavigate the globe alone nonstop but had to give up her chance at that record when she was forced to pull into a port at Cape Town, South Africa, for repairs to her boat.
Laurence said his daughter had all of the safety equipment she needed, including a cold water survival suit, life raft and bag with emergency supplies.
The area is one of the most difficult parts of the world to launch rescue operations. British solo sailor Tony Bullimore had to be rescued by an Australian navy frigate in 1996 after his yacht capsized during a race in which another competitor died.