From the Fallout 3 Wiki page
LinkBig trouble in big town - Robots(Note: This appears to have been fixed with the first patch for all platforms. A sentry bot appeared for me. I am not sure if a Protectron was supposed to have appeared as well but I was allowed to proceed with this portion of the quest by just fixing the sentry bot.)
The broken robots option is bugged (observed on all 3 platforms). When you go to find the robots, they are nowhere to be found in the junkyard. On the PC, to get them to reappear, you have to use the following Reference ID codes to get the robots to reappear.
Protectron - 0002d934 Sentry Bot - 0002c11b Since the player.placeatme command does not work with RefIDs, you have to go through this set of commands: (in big town, near the junkyard, press ~ to open the console, then type these in. After each line, hit enter)
prid 2d934 kill resurrect disable enable -
prid 2c11b kill resurrect disable enable -
This will move the destroyed bots to the Player's location, so it is advised to be standing in the locale where they should have been.
The bots may seem stuck in the ground, but as soon as they're "repaired", they will do their "get up" animation and properly stand on the ground.
Make sure you do hang around after doing this, for it won't be long before Super Mutants attack the town (4 total, you will get positive Karma when they're dead). Afterward talk to Red and she will thank you for having defeated the Super Mutants.