Wait until miss Vance comes back to senses and disables the force field (#57). After a short walk you'll be in a tunnel (#58) filled with Zombies and Headcrabs. You don't have any weapons on you, so leave them to Alyx. You can help her by throwing different objects at them.
Find a way between the cars and head to a narrow passage (#59). There's a door on the other side of this corridor, but you can't open it due to the lack of power. Turn right. Next to the barrel there's an Air Vent (#60) which you can use after opening it with a Grav Gun.
Once inside, crawl through. You'll reach a small room. Turn right. There's a Shotgun, a Pistol and some other useful stuff behind a fence. You can reach them with a Grav Gun, pulling them through the hole under the ceiling.
Approach the wired gate (#62) on the other side of the room. It's closed with a padlock - use your pistol on it. Find a fuse box a little bit further. Open it and power will be back on. Gather all other items and destroy the supply crates.
The way is blocked by a wrecked car (#64). Push it with your almighty Gravity Gun and enter the dark tunnel. It's filled with Zombies, but you don't have to fight them - Alyx will do just fine, you just point your flashlight at them. There's a battery and some ammunition to the left of the entrance. Head inside the tunnel and go between the cars. Dead Antlions (#65) show you the way to go.
There's a wrecked train at the end of the tunnel. Approach it for a second and immediately retreat, as one of the enemies will throw a grenade in there. You'll be back in the tunnel, where you'll be attacked by Zombie soldiers. They are much tougher than their predecessors, so support Alyx this time. Use explosive barrels laying here and there to help yourself. Go further and you'll encounter Antlions and Barnacles (#67).
You'll reach a small room (#68) where you'll find a fuse box (#69). Use it and go back to the tunnel. You might have noticed an electronically locked door before. This time it's open (#70). Go upstairs and deal with the spiders.
In the next room, being in fact one of the levels of an underground parking lot, there's a hole in the ground. It's being used by the Antlions as an entrance. You don't have enough ammo to fight them here. Instead of that, grab a green car (#71) with a Gravity Gun and throw it into the hole. Now turn the gauge (#72) to open the passage.
Go ahead, using the flashlight on the enemies. It's good to make use of Alyx and let her do the shooting, as your ammo is very limited. At the end of the way you'll see a door labeled Exit (#73). Before you use it, look for a crack in the wall (#74) on the other side of the tunnel. There's a med kit and some ammo.
Use the staircase and you'll on the fifth level of the parking lot (#75). There are some Zombies here, but they're not the real problem. There are three holes in here, all of which serve as entrances for the Antlions. As you might have guessed, now it's up to you to close them all.
First grab a red car (#76) and throw into the first hole. The second car is on the first floor. You'll get to it using the plank (#77). Use the Grav Gun to push the vehicle to the lower level (#78) and use it to cover a hole.
The third car is also on the first floor, but in order to get to it, you'll have to go even higher - using another plank (#79). Find a ruined exit (#80) and jump down from it. When you approach the car (#81), a Zombie will rise, so have your shotgun ready. After shooting his head off, use the gauge to open the gate. Throw the car down and use it to block the last hole. Look around the room for some supplies now.
Run downstairs and join up with Alyx. There's another gauge nearby (#82) that will open the passage. Go through the door and use the stairs. You can go right through the door (#83), but it's useful to go lower and find a supply crate. You'll eventually reach flooded corridors (#84). Go through them, steadily eliminating the enemies.
After a short walk you'll find yourself in a room with an elevator (#85). Alyx will try to call it, but electricity will fail you one more time. The girl will suggest that the nearby cable should lead you to a fuse box. Go along it until you see a locked door (#86) (look around for some ammo).
Open the door with your Gravity Gun and go inside. The fuse box (#87) is to the right. The elevator will work now, but some enemies will appear next to it as well. Get through them and enter the elevator (#88).