Natasha Romanoff died in Avengers: Endgame, and Marvel Studios made it clear a while ago that the Black Widow movie is set between the events of Captain America: Civil War and Avengers: Infinity War. As well as exploring the character's past, the hope is it will also set up some future stories through the introduction of characters like Taskmaster and Yelena Belova.
In the Black Widow: The Official Movie Special Book (which is now on sale from Titan Books), co-producer Brian Chapek shared some more details on the movie's setting, revealing some specifics about when we pick things up with Natasha.
"This film takes place immediately after Captain America: Civil War," he confirms. "That movie basically ends with Tony Stark telling Natasha that, 'They're coming for you.; What that means is that she's just broken the Sokovia Accords. She's gone against General Ross. The Avengers are disbanded and she has no one. That's where we find her at the beginning of this movie."
"She's desperate to escape U.S. soil, and when she does evade Ross she almost has an opportunity to start over. But she quickly realizes that there are darker forces out there in the world that may have been waiting for this opportunity, and this forces her back into action. Unfortunately, she doesn't get to have a retirement, which is probably something she wished she could've had."
Based on what Chapek is saying here, it sounds like Black Widow manages to escape from General "Thunderbolt" Ross and finds peace before being forced to suit up and return to action. Whatever happens next is clearly what leads the hero to join Captain America and his Secret Avengers.
Stay tuned for more details on Black Widow.