What was happening in Lanka during Mahabharata ?
We all know that Rama killed Ravan and established Vibhishan as the King of Lanka. However we know very little about what happened after that. Vibhishana is actually a chiranjivi which means he wont die just like Hanuman. Which enabled him to live a real long life. As a consequence when Yudhishtira performed Rajsuya yagnya, Vibhishan still was the king of Lanka. Sahadeva travelled the length of the country and reached the southern most part of the country.
Vibhishana learning of this, sent his official from lanka via boat to the mainland and offered various gifts to Sahadeva and offered his best wishes for the Rajasuya Yadnya. Vibhishana did not travel to Indraprastha nor did he participate in the war nor sent anyone to participate. Very few Rakshasas participated in the Mahabharat war as this was mostly seen as the fight among the men.
Unfortunately we do not know much details about what happened to Lanka post Vibhishana. His rule was far too long and hence we do not know who succeeded him and how Vibhishana disappeared.