In the seventh episode of Stranger Things season 4, it was confirmed that the Upside Down was stuck in 1983. Though it has not been revealed why it is stuck but there can be many reasons.
The Upside Down is a mysterious alternate dimension parallel to the real world. When a gate is opened between the two dimensions, threats from the Upside Down sneak into Hawkins. We don't know much about this world but the main characters have been investigating it which has given us some answers.
Will Byers was the first person to spend some time in the Upside Down and make it back to the real world. Though in season 2 he was confirmed to be the new host of the Mind Flayer, it confirmed that humans could travel throughout the alternate dimension. In Stranger Things season 4, episode 7, "Chapter Seven: The Massacre at Hawkins Lab," Steve, Nancy, Robin, and Eddie entered the Upside Down through the "water gate". Upon arrival, they had to face the Demobats. Nancy suggested going to her house to retrieve her guns. While there, she discovered that the guns hidden in her room weren't there. This was because her guns hadn't existed yet, realizing through the diary in her room that the Upside Down was stuck on November 6, 1983.
This date is very important in Stranger Things for several reasons. First, it was the date that Dr. Martin Brenner asked Eleven to make contact with a "monster" with her powers. In doing so, she accidentally opened a portal between her world and the Upside Down. On the same day, Will Byers was also taken by the Demogorgon into the Upside Down. It was likely the first time that the elements from both dimensions mixed. This was probably the reason that locked the Upside Down on November 6, 1983, in some kind of endless time loop.
In an interview with Variety, the Duffer Brothers were asked why time stopped in the Upside Down with Will Byers' disappearance. Matt Duffer responded, "That is a very good question. That is actually one of the few questions that we've raised this season that isn't going to get answered in Volume 2."
He explained, "But that's setting the stage for some big reveals in our final season. We've said this before, but Netflix, midway through Season 1, they went, 'Can you just please explain to us what the Upside Down is, and where these monsters are coming from — all of this stuff?" The co-creator continued, "So we wrote this document — we spent like a week with our writers just writing out all the mythology and that really is where we really defined what the Upside Down was and where these monsters were coming from and what it all meant. And so this season, we are giving the audience a lot of information, but there's still mysteries about the Upside Down, and that is a big one. But we really wanted to hint at it this season because it's going to play such a huge role moving."
It looks like we will get some answers in Volume which will arrive on July 1.