You can either acquire this quest from a distress signal near the National Archives or from speaking with Reilly herself in the Underworld. She is in the medical station. You will need to convince the doctor to wake her up (speech challenge) or try yourself (medical challenge). She needs you to rescure her rangers from on top of the Statesmen Hotel. You can only get inside from the second floor, through a walkway from the Our Lady of Hope Hospital. Head to the hospital, through the walkway and to the roof of the hotel. The rangers need you to find a Fission Battery to repair the elevator. Head back to the room that accessed the roof and find a robot in a closet on the first level. Search it for the battery. You can of course grab this on your way instead of backtracking after. Once the rangers fix the elevator, you now just need to fight your way out of the hotel.
See Reilly's Rangers for more specific info.