65 ILCS 5/1-1-9
If a municipality which is a home rule unit under Section 6 of Article VII of the Constitution by reason of having a population of more than 25,000 suffers a loss in population so that its population determined as provided in Section 1-7-2 is 25,000 or less, such municipality shall continue to have the powers of a home rule unit until it elects by referendum not to be a home rule unit.
Unless such a referendum is held sooner, or such a referendum has been held within the 2 calendar years preceding the year in which the population is determined to be 25,000 or less, the municipal clerk shall certify for submission to the voters of the municipality at the next general election following such determination of population, in the manner provided by the general election law, the proposition of whether the municipality shall elect not to be a home rule unit.
65 ILCS 5/1-1-9