So I'll start from the beginning. Once you wake up and exit the starting room in B1, you loop around from the middle left, hugging the bottom wall, traveling right. Then hug the wall up and keep following the path until you reach the non-working elevator. Pass it and find the staircase on the top left to travel down. You'll be in B2.
Once in B2, you can look at a few things before entering the staircase on the right side. Once you go down that staircase, you'll be in B3.
B3 is dark. There's several rooms you can find things in and weapon. But the nessary thing you need is a battery, which can be found in the room closest to the right bottom side. Once you grab that, you can plug it into the terminal in room on the top left of B3. Doing that activates the elevator.