The National Park Service believes the Washington Monument's lighting system clock was likely to blame for the outage Tuesday night.
Officials say that when electricians arrived Wednesday morning, the lights were on. Crews are working to correct the problem.
The lights went out at around 7 p.m. Tuesday and affected the lights that illuminate the D.C. landmark. The blinking red aircraft warning lights at the top of 555-foot monument, however, were still working.
The monument has been closed to visitors since August because of ongoing problems with its elevator, which carries visitors to an observation deck near the top of the 555-foot tall monument. It usually draws 600,000 visitors a year.
In December, officials announced a $2 million to $3 million project to modernize the elevator. The monument is expected to reopen to visitors in 2019.
The lights are expected to be on this evening as normal.