[ TL:DR - What is happening in Myanmar ]
For almost the entire history of modern myanmar, they are ruled by the military until around 10 years ago, where the military start to do reforms and allowed democratic elections

The National League of Democracy (NLD) won power in the last election (4-5 years ago), sweeping 90+++ percent of the votes. (the first election, NLD is disqualified, allowing military backed party to win). However, democratic elected officials only account for 75% of the lower and upper house.

In the military drafted constitution, the military will always get 25% of the seats automatically, and the military and police related ministries will always be under the military control.

Thus essentially, ALL THE GUNS are with the military and with sufficient numbers to forever block constitutional changes to change the status quo.
So in this election, the military claimed there is election fraud. But like what happened in the US, election commission simply brushed it away without remedy or even entertaining the military's allegation.

Since being the only people with "real power", they took over by force.