Hundreds of artists, including David Guetta, Lost Frequencies, Swedish House Mafia and Charlotte De Witte are expected to perform.
The local fire service has declared the site safe and a decision will now be made about whether to demolish the structure before the festival begins on Friday.
The campsite, known as DreamVille, has opened and so far organisers say this weekend's event will continue.
In its latest post on Instagram on Thursday, Tomorrowland said: "It is impossible to put into words what we're feeling."
It added that the Orbyz main stage "wasn't just a stage... it was was living breathing world."
Organisers said they had worked through the night to come up with solutions. A meeting was held with safety experts and members of local government on Thursday morning to discuss a contingency plan.
The mayor of Rumst, just north of Boom, told local media that another meeting discussing more ideas for an alternative to the main stage would be held in the afternoon.
"Cancelling the festival completely is the last thing we want to do," Jurgen Callaerts said.
There are 14 other stages at the festival, all much smaller than the main stage.
Further details are expected to be released later on Thursday.