Brock says Conaway is in a coma in an Encino, Calif., hospital and his recovery is uncertain. The name of the hospital was not disclosed.
As part of the reality show "Celebrity Rehab" in 2008, Conaway discussed his addiction to drugs and alcohol.
His manager described Conaway as a "gentle soul" but one who has been unable to "exorcise his demons."
Conaway appeared in the 1978 movie musical "Grease" and starred in the TV series "Babylon 5."