This was a movie about a couple that cannot have kids, so they create a robotic child. But there were some kind of natural disaster and the human race is extinct, and when other people came they found the robotic child, and he was looking for his mother.
The responsible person told him that he can bring his mother back for one day only if he could bring him her DNA, so the boy remembers that one day he grabbed her hair and start looking for it and his mother came back to life.
This the 2001 Spielberg film A.I. Artificial Intelligence, the story of the life of David, an artificial boy created to be a companion to childless couples.
Quoting from the plot summary on Wikipedia:
Two thousand years later, humanity has become extinct and Manhattan is now buried under glacial ice. The Mecha have evolved into an advanced form, and a group of them called the Specialists have become interested in learning about humanity. They find and revive David and Teddy. David walks to the frozen Blue Fairy statue, which collapses when he touches it. The Specialists reconstruct the Swinton family home from David's memories and explain to him, via an interactive image of the Blue Fairy, that it is impossible to make David a real boy. However, at David's insistence, they use their scientific knowledge to recreate Monica through genetic material from the strand of hair that Teddy kept. This Monica can live for only one day, and the process cannot be repeated. David spends his happiest day with Monica, and as she falls asleep in the evening, she tells David that she has always loved him: "the everlasting moment he had been waiting for", the narrator says; "David falls asleep as well and goes to that place 'where dreams are born.'"
This is almost certainly AI: Artificial Intelligence.
David's parents already have a child, but he's in cryosleep due to an untreatable illness. At the end of the film David is (or rather the super-intelligent evolved robots who find him are) able to resurrect his adoptive mother because he cut off a lock of her hair.
