Amid the coronavirus pandemic, sportsbook operators, like businesses around the world, are facing challenges they've never considered before, including: What if there are no games to bet?

On Thursday, the NCAA canceled the men's basketball tournament, the biggest betting event in U.S. sports. Billions of dollars are estimated to be wagered on March Madness annually through brick-and-mortar and online sportsbooks, as well as office bracket pools.

The NBA, NHL and Major League Soccer have suspended seasons, and Major League Baseball is halting spring training and delaying the start of its season. On Thursday, after many college basketball conference tournaments were canceled, only a handful of events remained listed on oddsboards at sportsbooks.

In a statement, the NCAA said the decision to cancel the basketball tournaments, both men's and women's, along with all remaining winter and spring championships is "based on the evolving COVID-19 public health threat, our ability to ensure the events do not contribute to the spread of the pandemic, and the impracticality of hosting such events at any time during this academic year given ongoing decisions by other entities."

The NCAA men's basketball tournament had been played uninterrupted since 1939.

Sportsbooks and casinos around the nation remained open heading into Friday, and operators were not expecting any stoppages, but they also acknowledged the unpredictability of the situation and the reduced number of sporting events.

Nevada Gaming Control declined to comment when contacted by ESPN, and the New Jersey Division of Gaming Enforcement did not respond to a request for comment on any guidance being given to gambling licensees.

Losing the NCAA tournament, at least for now, is the most costly blow to the sportsbook business. March has routinely been one of the most lucrative months of the year for bookmakers. Sportsbooks compare the betting handle on the first four full days of the NCAA tournament to how much is wagered on the Super Bowl. More than $154.6 million was bet on Super Bowl LIV between the Kansas City Chiefs and San Francisco 49ers at Nevada sportsbooks, with the books winning a net $18.8 million.

While the majority of sports bets are made online, the NCAA tournament does draw large crowds at sportsbooks. As of Wednesday, there are 16 states that have taken a legal sports bet. In Las Vegas, hotel occupancy is typically high for the first week of the tournament.

"It's our busiest week of the year," John Murray, executive director for the SuperBook at Westgate Las Vegas, told ESPN. "In terms of ticket counts and crowds, that's our busiest week of the whole year, the first week of the NCAA tournament. It's a big concern, but every sportsbook in the country is in the same boat. There's nothing we can do to control it. We're just hoping for the best, while keeping the safety of our guests, our staff and everyone involved our No. 1 priority."

With no tournament, bets on the odds to win the NCAA championship will be refunded at most sportsbooks. Books are taking a wait-and-see approach on how to handle futures bets on the leagues that are suspended, like the NBA.

"We're waiting until leagues either cancel the season or declare a winner," Kevin Fulmer, vice president for Caesars Entertainment, told ESPN. "That's how we're going to decide futures bets."

After the NBA announced its decision to suspend its season Wednesday, sportsbooks were taking varying approaches. Bookmaker William Hill U.S. said Thursday that it was continuing to take bets on the NBA's odds to win the Western and Eastern Conferences, for example. Caesars Sportsbook, on the other hand, halted betting on NBA futures.

Season-win total bets -- over/under propositions on the number of games a team wins during a regular season -- are more complicated, and house rules can differ from sportsbook to sportsbook. Caesars Sportsbook, for example, requires NBA teams to play at least 81 of 82 games for season-win bets to be considered action. The SuperBook requires teams to play all 82 games.

Odds on events such as the Super Bowl and World Series are still on the board at most sportsbooks, and the PGA Tour and NASCAR continue, albeit without spectators.

"There will be futures markets available [to bet]," Fulmer said.