The developer of the game said technical issues prevented users from logging in.
Things were so bad, the game's developer will refund the $20 ticket price and give players $100 in PokéCoins in their Pokémon GO account.
Players lined up outside Grant Park's entrance at 6 a.m. Saturday, four hours before the festival opened.
Tickets for the event -- the first of its kind -- sold out in less than 10 minutes. Some people paid upwards of $450 on eBay for tickets after they quickly sold out.
Some traveled from around the world, just to play Pokemon GO.
Details of Pokemon GO Fest have been limited, but trainers have been promised never-before-seen Pokemon.
In a statement, Niantic, the software company that developed Pokemon, said: "Today at Pokémon GO Fest in Chicago, technical issues created problems for a large number of players attending the event. From everyone at Niantic, we apologize to all of the Trainers who came out to Pokémon GO Fest today. Although we were able to solve many of the technical issues, we were not able to offer every attendee a great experience."
Niantic also said they would try to extend the Pokemon experience through Monday.
They said: "Special Pokémon, Eggs, and check-in PokéStops appearing during Pokémon GO Fest have had their range increased to a two mile radius surrounding Grant Park through Monday morning, July 24. These Pokémon and Eggs will only be visible to Pokémon GO Fest attendees who validated the QR code they received when they entered Pokémon GO Fest. Attendees who were unable to validate their QR code during the event can do so through the special PokéStops through Monday morning."
However, after the app crashed, players said they felt "totally ripped off" because of the technical problems.
Rob Schuyt, of Minnesota, said he spent hundreds of dollars for a car, hotel and parking. He said event organizers "failed miserably."
"Complete and utter failure," he said. "And I think it was totally predictable by Niantic. They should have known you can't have 20,000 people in a two-block area all trying to connect to LTE and their servers. It's just nuts."
"My son was crying the whole time he was here," said Mariza Jimenez, who traveled from Wisconsin.
Jimenez brought her son, Andrew, for his 6th birthday. He was playing and suddenly got a "network error" message and then couldn't continue playing.
"I mean what kind of birthday is this? This is ridiculous. They promised so much and they couldn't even, they couldn't hold up to their word," Jimenez said.
WHAT IS POKEMON?
So if you're feeling out of the loop, we'll fill you in.
Pokemon Go is an augmented reality game that allows "trainers" to catch Pokemon (short for "pocket monster"). Players can collect virtual items at floating blue cubes called "PokeStops" and participate in Pokemon battles with other players at "Gyms."
Many of the PokeStops and Gyms are at real parks, churches or well-known landmarks.
The app models the popular "Pokemon" TV show and video games that followed the journey of Ash Ketchum, a 10-year-old fictional Pokemon trainer who searched far and wide to catch 'em all.
Ash's end goal? Become a Pokemon Master.
"I think there needs to be a lot of praise to Niantic for developing a game that made so many people in America get outside, be more active, and experience our parks," said Brandon Omernik, 23, a fan who drove from Wisconsin to attend the festival in Chicago. Niantic is the company that created the mobile game.
In April, Niantic announced the game had more than 65 million active users each month. And just a few months before that, the app surpassed 650 million downloads.
Over the past year, Niantic has updated the app numerous times to keep active users interested.
One of the most celebrated updates added Pokemon characters from the second generation, including Togepi and Phanpy. The first generation included fan favorites like Charmander, Squirtle and Bulbasaur.
CNN Wire contributed to this report