угрожать is to threaten and it's about people, not really about things or abstract objects, maybe there are some exceptions to this, but угрожать is usually to day that you harm someone if they don't do what you want them to do

Вова угрожал Денису, что расскажет всем о его грязных делишках

грозить is to threaten but, it's mostly about abstract things, it's when something can cause some harm to something or someone, or is likely to destroy something or someone.

Мне грозит увольнение - Увольнение can fall upon the person
Некоторым диким животным грозит вымирание - Extinction threatens some wild animals