According to Harbaugh, Kindle was at a "function" with some friends.
"He stayed overnight, got up in the middle of the night and took a wrong turn," said Harbaugh, who spoke with Kindle's father but hasn't been able to reach the rookie, according to the team's official Web site. "It was pitch black where he was at -- he couldn't see where he was going –- and he fell down two flights of steps, so that's what we know."
According to NFL Network insider Jason La Canfora, the Ravens still intend to sign Kindle, who can't travel to Baltimore for at least three weeks.
The Baltimore Sun reported that general manager Ozzie Newsome has consulted with the NFL management council as to how to proceed in talks with the team's only unsigned rooke.
"I know one thing, we're going to be very careful with that. You're talking about a head injury and playing football," Harbaugh said. "That's not something we want to mess with. From a football standpoint, we'll move on with the guys we've got."
Ravens rookies reported to training camp Monday at McDaniel College in Westminster, Md. Veterans are scheduled to report Wednesday.
Kindle, who's 6-foot-3 and 250 pounds, played outside linebacker and defensive end at Texas and was named a first-team All-America by The Sporting News in 2009. The Ravens took Kindle in the second round (43rd overall) of April's draft despite concerns over a right knee injury.
The team will have to take a wait-and-see approach with Kindle's recovery.
"It's like any injury. If a guy gets hurt, you move on to the next guy, so we'll be preparing without Sergio," Harbaugh said. "You're putting your team together as if he's not out there, and that's the same as with any injured player. Then if at some point in time, if he can come back and play, that will be a great bonus for us. He'll have a lot of catching up to do at that point."
The Associated Press contributed to this report.