Sheriff's spokeswoman Paula Miller said a faulty electrical transformer knocked out power to the courthouse early Thursday.
"The transformer is blown. It's going to be an all-day, all-night repair," Miller said. She said it wasn't clear what caused the transformer to fail but that it posed no security risk.
The trial was expected to resume Friday morning.
Muhammad, who stunned the court Monday when he demanded to represent himself just before opening statements Tuesday of his capital murder trial, on Wednesday changed his mind and rehired his defense lawyers.
"You don't know how emotional it is for a lawyer with death on the table to be sidelined in deference to a defendant's right to represent himself," said Muhammad's lawyer, Jonathan Shapiro.
Muhammad, whose face is badly swollen from a chronic toothache, did not spell out his reasons in open court but assured the judge it had nothing to do with his health.
Though Judge LeRoy F. Millette Jr. said Muhammad represented himself competently, legal experts said he probably inflicted heavy damage on his case with his opening statement. During testimony, many of Muhammad's objections were overruled, and prosecutors objected to the way some of his questions to witnesses were posed.
"I think he panicked," says CBSNews.com Legal Analyst Andrew Cohen. "Just before his trial he wanted control. Once he got control he realized how difficult it was to actually maneuver the intricacies of the law, so he gave up."
Muhammad, 42, is charged in the slaying of Dean Harold Meyers, killed by a single bullet at a Virginia gas station during a three-week spree that left 10 people dead in the Washington, D.C., in October 2002.
Wednesday's testimony, however, focused on two shootings that preceded that spree.
Muhammad Rashid, a liquor store employee, identified Muhammad's fellow suspect Lee Boyd Malvo, 18, as the man who shot him without saying a word as he locked up the store in Brandywine, Md. on Sept. 15, 2002.
Rashid said he fell to the ground and played dead so his attacker, who took his wallet out of his pants pocket, would not shoot again.
Rashid hung his head while prosecutors played his call to 911, in which he pleaded for help for six minutes, repeatedly telling the dispatcher "I am dying" and moaning in anguish.
Another witness, Kellie Adams, testified about the Sept. 21, 2002, attack outside a Montgomery, Ala., liquor store that killed her co-worker, Claudine Parker, and left her breathing through a tube.
Adams said she never heard the gunshot that pierced the back of her neck and came out through her jaw. "The whole left side of my face was just kind of splayed open," she said.
Adams only saw her attacker's legs, but a witness identified Malvo as the man he chased.
"That's him," a tearful James Gray said after Malvo was brought into court again. Upset, Gray covered his face and hands.
Gray testified that he came face-to-face with the attacker and got a "very good glimpse" of him.
Gray added: "His eyes were big and they were round. They looked wild, like he was in some kind of frenzy."
Malvo is scheduled to go on trial separately in Chesapeake next month for the Oct. 14, 2002, slaying of FBI analyst Linda Franklin. He also faces the death penalty if convicted.
Malvo's lawyers gave notice this month that they plan to present an insanity defense. In response, prosecutors on Wednesday asked that his trial, set to begin Nov. 10, be postponed for a month to give their mental health expert time to examine and evaluate Malvo. A hearing on that request was scheduled for Thursday afternoon.