Crimea is not an ethnically homogeneous state. There are three significant populations in Crimea:
1. Russians. The majority of Crimea is Russians; they came from Russia and made Crimea their new home (although most Russians don't consider Crimea (or Ukraine, even) as separate countries; according to them, they are still a part of Russia). They are vehemently in favour of Russian rule and are against the recent events that have happened in Ukraine.
2. Ukrainians. For the most part they have pro-Western ideals and are upset about Russian intervention. Crimea is a part (simplifying here) of Ukraine and has been since 1992. By retaining control of Crimea, Ukraine has a pretty strong bargaining chip against Russia. This is the ideal solution for the Ukrainians of Crimea.
3. Crimean Tatars. Tatars are native Crimeans. They are a tiny population of Crimea because of the ethnic cleansing that happened in the 1940s. Most Tatars that were deported died on their way to Central Asia. Due to the small size of the Tatar population, nobody gives them any mind and they are generally ignored while Ukraine and Russia fight over their country.
The problem should be easy to see. Crimea "belongs" to Ukraine. Its population is mainly Russian, who wish to be a part of the Russian Federation. It lies in a geographically important spot, mainly for economic and trade reasons. There are countering ideals of pro-Western and pro-Russian people. And the most important part: Ukraine, Crimea and Russia have a history of being very violent locations.
All of this baggage exists in this part of the world. But let's not forget about the EU, NATO and the US. There is no easy solution to this.