I cant find an explanation for the meaning of 'nous est arrivé'

Chirag, 

which exercise or lesson does this come from ? What is the full sentence - you have only provided the indirect object and verb ?

Without the details, this looks, rightly or wrongly, like it could be homework. 

The meanings and conjugation of ‘ arriver ‘ can be found in online dictionaries :

 https://www.wordreference.com/fren/arriver