A power vacuum
is an expression for a political situation that can occur when a government has no identifiable central authority.
During or following a civil war there is often a power vacuum of some sort. For example, the war-torn nation of Somalia is currently mired in a power vaccum of sorts, with no central government or president exercising control over the supposed "Republic of Somalia".
A power vacuum can also occur following a constitutional crisis in which large portions of the government resign, creating unclear issues regarding succession to positions of power.