What should a person do if they forget whether they have wudu or doubt their state of wudu?

If a person is certain that they have performed wudu but doubts whether they have invalidated it, that person is considered to be in a state of wudu. On the other hand, if someone knows for sure that they have invalidated their wudu but later doubts whether they performed wudu again, they are considered to be without wudu. This is because something that is certain is not nullified by doubt. (Mevsılî, el-İhtiyâr, 1/11; İbn Âbidīn, Reddü’l-muhtār, 1/119)

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu