Fey, 41, famously hired Lohan to play the lead role in the 2004 movie Mean Girls, and she was scheduled to participate in the actress' TV comeback.
"Tina was committed to doing the monologue with Lindsay but both of her children got sick," Fey's rep tells The New York Daily News. "She was unable to participate as planned."
Though Lohan, 25, received mixed reviews for her performance, she did give Saturday Night Live a huge ratings boost. It was the second most-watched episode of the show in its 37th season (behind Charles Barkley's January 7 appearance) and the No. 1 show in its time slot.
Lohan also impressed the comics of Saturday Night Live. "I think she did great. There were a lot of expectations, and I hope we met [them]," Bobby Moynihan, 35. tells MTV News. "I think she really wanted to do a good job, and I think she pulled through."