The simplest way to explain what has happened…
A country divided nearly fifty-fifty between ethnic and culturally Russian and Ukrainian, voted in a Russian leaning President, who promised to work toward moving toward Europe.
As the European push neared signatures he went against his campaign pledges, after strong encouragement from Russia.
This began months of protests(in bitter cold), mainly by ethnic Ukrainians, but also somewhat by ethnic Russians, who wanted to move toward European freedoms while staying friendly with Russia.
He then increased his and the states power.
Once violence was used by the President, the parliament was about to arrest him, and he ran to Russia.
Parliament elected a temporary President and scheduled elections.
Russia expanded out of a Naval base they had on lease for decades to come.
They sent in and are sending in more troops and kicked Ukrainians out of several of their military bases.