The mother of three was last seen walking away from her home, police said.
Family members said she had been residing there, with an unnamed man, for two months, and left without shoes, her cellphone or her car.
She was to have picked up one of her children, living in nearby Hinsdale, Ill., the Monday following her disappearance, said Fellenbaum's mother, Deborah Barr of Chicago.
"She was supposed to show and take him to school. Never showed," Barr said.
About 50 friends, many Fellenbaum's high school classmates from Chicago, arrived in her subdivision neighborhood Sunday, to knock on doors in the area and walk through nearby fields in search of clues, The Herald-News newspaper in Joliet, Ill., reported Monday.