Answer:
Authoritarian rule or dictatorship
Step-by-step explanation:
A successful democratic system is one where participation is wide as possible. So, if the state is ruled by just one party, that could mean that this specific governement is using different methods, some of them non-ethical or even illegal, to maintain itself ruling. In that case the single-pary rule would be due to dictatorial practices.
Nevertheless, a single-party rule might indicate an hegemony of that party because the electors are happy and satisfied with their government.