Russia gave Ukraine the Crimiea, now they want it back?
Technically, they "gave" it to the Ukrainian Soviet Socialist Republic (which basically belonged to Russia), not Ukraine. It's a bit like putting assets in your wife's name for tax purposes, then she divorces you and keeps them.
The people of Ukraine have asked for help from the West so should we turn our back on them? If the Falkland Islands were invaded by Argentina, and the poeple living there asked us to protect them should we turn our backs on them?
The difference is though that the people of Crimea don't want to be part of Ukraine, they want to either join or create stronger ties with Russia, not the EU.