What should I do now that I no longer have milkweed?

You should note that it was mowed in the notes for that site, and then hopefully when the milkweed comes back it will have even younger plants that are more attractive to monarchs. Milkweed typically grows very quickly, so you might even start to see small growths of it coming up above the rest of the plants in your area fairly soon after mowing. If no milkweed comes back within a few weeks, you won't be able to monitor – at least until next year.