The Hokkan Ice Age refers to the period of eternal frost that covered Hokkan. It is one of the central plot devices in the prequel, Fushigi Yuugi Genbu Kaiden, and is the very thing that has forced Takiko to summon Genbu. Its brewing has also delayed the ongoing Battle at Touran, forcing the Kutou Army to temporarily retreat and delay the battle until it stops.
It starts at the end of Chapter 36, as pointed out by Teg, but it becomes much more serious in Chapter 37. Throughout the mid-prequel, signs of abnormal temperature drops have already been hinting at the disaster. As time passes, the weather becomes too uncomfortable, even for the well-adapted Hokkan citizens. The ice age was predicted long before by Temudan Rowun, and it was stated that it is unpreventable and goes along the course of nature. It is the result of the rotting Hokkan land and the endless wars that have defaced the country.
The brewing of the eternal frost has done quite a damage, as shown in Chapter 38. Citizens have been killed with the projected hail, the even more sudden drop of temperature is freezing everything, and even the palace walls are torn down. Fortunately, in Chapter 39, as Genbu is summoned, the catastrophe is stopped.