You should find the advice useful in this process. - MitchellMc
If you feel that your lawn has been overtaken by weeds, it is worth starting from scratch and following Mitch's advice above.
If you have some grass left and decide to try recovery first, I recommendYates 2.4L BuffaloPro Weed 'n' Feed Hose On Weed Killer. This is safe for buffalo and other common lawn types. I then propose an aggressive campaign of manual weed removal. It doesn't have to be all in one go: you can complete the weeding in sections as time permits.
I then propose aerating the soil and topdressing it so that you can prepare it for reseeding or patching it with instant turf. - EricL