Perhaps you download The Pitt when you were on the last quest and then you have not finished, I will not put any Spoilers, but you cannot continue after this
It is near the Jefferson Memorial.
The Jefferson Memorial and in parts of the Capital Wasteland. http://fallout.wikia.com/wiki/Fallout_3_map
As Fallout 3 players will know, it was in the Jefferson Memorial, Washington D.C, made by the Lone Wanderer's father
well it cant be fxed but its problably that your disc is either dusty or scratched
Thier are simply to many to count but i will give you a couple any ways. One place is in Sergrave Holmes home in rivert city. Another is in the Jefferson memorial on top of the thing you would place the virus into. The last is in an area in Jefferson memorial that you find whil exploring.
It depends on which time you are there. The first time (which is my guess) you need to make your way to the Rotunda and you will eventually find a set of notes from James, where you need to read through them to discover where he went.
Really, there is no way to actually "beat" it, although you can take different paths to see the ending of the game. You would have to take the missions about finding your father and in the citadel, then you would come to a mission about taking back the memorial, killing all the mutants and go back to purifying the water by activating the water purifier that is in the center of the memorial.
seeing that point is the end of the game nobody knows for sure.
travel east from Anchorage memorial.
the broken steel expansion continues after the purifier final quest on the original game, you can level to 30 without completing the main questline.
The Citadel is found after your father James is killed and Dr. Madison Li tells you of a secret tunnel then as you go through the Jefferson memorial to the Citadel which is built into and beneath the ruins of the Pentagon in Arlington, Virginia. It is located on the west end of Arlington Memorial Bridge, northeast of Arlington Library.