Crews spent Monday peeling back carpet, airing out furniture and repairing ceiling tiles inside the Willard Hall Education Building.
Authorities say someone went on a destructive spree overnight Sunday.
Police say the suspect turned on a shower and just walked away, flooding the building and derailing classes for an entire day.
Now Investigators are asking for help finding whoever is responsible.
Students were once again walking in and out of Willard Hall Tuesday, but on Monday the building was closed.
"I got there, and there was a class standing outside with one of the professors. They said the doors were locked and nobody could get in. And then they said classes were canceled," said Amanda Toto of Hopewell, N.J.
"I came around 8:30, and it was locked," said Joanna Hickmott of Washington D.C.
"We got an email about 5 minutes before class started that it was going to be moved to the green, so I ran to class," said Calla Masters of Newark, Delaware.
All classes inside the building were either moved or canceled. Police said sometime between 10:00 p.m. Sunday and 5:00 a.m. Monday someone got in and turned on an emergency shower outside a lab on the third floor. Water gushed from the shower at the rate of 20 gallons a minute.
Police think the person who did it thought it was a prank.
"It's not harmless. Thousands of dollars of damage and disruption," said University of Delaware Police Chief Patrick Ogden.
All of this happened heading into finals week.
Police are talking to student groups who were using the building. Ogden said they are using all the tools at their disposal to find the person responsible.
"There are cameras strategically located throughout campus," he said.
Clean up began almost immediately on Monday morning. Crews had to dry out the second and third floors.
"We got it pretty much situated in the afternoon and it dried overnight. Classes are back in session with a minimal impact," said the school's Director of Environmental Health and Safety Michael Gladle.
Now students are focused again on passing their finals, as police hunt down the person who derailed classes for a day.
Anyone with information on the crime or the culprit is asked to call University of Delaware Police at 302-831-2222.