The singer-songwriter scrapped rehearsals on Monday (October 8) in order to visit her grandfather James in Kent, The Sun reports.
Marjorie Spraggan passed away on Sunday (October 7), a few hours after watching her granddaughter make it through to the second round of the ITV competition.
"My nanny was a great woman and she and granddad were together for more than 50 years so I really want to be around to support him and my family at this time," Spraggan said.
"My granddad is an amazingly strong and kind man and they loved each other very much. She's at peace now and that's comforting to know.
"She was so proud to see me singing and I'm glad I made her smile. She'll be greatly missed."
Lucy Spraggan returned to X Factor rehearsals on Tuesday afternoon.
A show source said: "We are giving Lucy as much time at home with the family as she needs at this sad time."
Spraggan recently predicted that Ella Henderson will be crowned the winner of this year's X Factor.
Lucy Spraggan talks to Digital Spy: