Are there any LOD generators for Fallout 3? For some reason all of my LOD in the DC ruins has gone away =/

There is an LOD generator in the GECK. Generating Object LOD is problematic for some users (including me) as many required .tga files are seen as missing.

If you have many mods enabled then there is always the chance that one of them has buggered up the LOD for the DC ruins. FO3Edit can help you find out if that is the case.

I remember a utility for Oblivion which did generate LOD. It didn't generate it for the landscape though, only visible when distant statics.