The 32-year-old actress on Monday told reporters via cell phone that she was "absolutely heartbroken" not to be able to join them in Italy, where "Proof" is one of 19 films vying for the Golden Lion award, the festival's top prize.
Her flight from New York was forced to turn back because of technical problems.
"I had an absolutely, extraordinarily unpleasant evening last night trying to get there," she told reporters, according to AP Radio. "I had all kinds of plane problems."
The festival "was going to be the highlight of my summer," she added.
In "Proof," an adaptation of the Pulitzer Prize-winning play, Paltrow plays a woman who worries she's inherited her brilliant father's madness after his death. Anthony Hopkins, Jake Gyllenhaal and Hope Davis co-star in the movie, set for release this fall.