Bynes, 27, starred in "The Amanda Show," as well as the films "Hairspray" and "What a Girl Wants," but her last major screen credit was a supporting role in 2010's "Easy A."
Her once-bright career has been eclipsed in recent years by her bizarre Twitter rants and media reports of her erratic behavior and brushes with the law.
"I worked on Amanda's show when she had her show," Cannon, 32, said on "The Howard Stern Show" Tuesday.
"That's like my little sister. To see someone you'd seen at 9, running around with a snotty nose, and now to see her like that is like: 'Wow, man! That's sad.' I tried to reach out."
Stern asked Cannon if he tried to contact Bynes' manager to which Cannon replied, "She doesn't have any of her people any more."