Carrie Fisher, 60, has reportedly suffered a 'massive heart attack' whilst travelling on a flight from London to LAX.
US news site TMZ reports that the plane that Carrie was on landed in America at noon and that the heart attack happened 15 mintues before the plane landed. A medic who was travelling on the plane administered CPR.
A law enforcement official also told NBC that her condition was "not good".
People who were on board the plane have taken to Twitter to explain what happened, including actress Anna Akana.
She tweeted: "Don't know how else to process this but Carrie Fisher stopped breathing on the flight home. Hope she's gonna be OK.
"So many thanks to the United flight crew who jumped into action, and the awesome doctor and nurse passengers who helped."
Carrie was busy in the run up to Christmas promoting her book The Princess Diarist, an intimate story of what really happened behind the scenes when she was filming the first Star Wars movie and was in London to film an interview for the Graham Norton Show.
We're sending all our love and prayers to Carrie and her family.