Question

how did the collapse of the western roman empire influence Justinian reign?

Answer 1

The collapse started the Justinian reign. It was the start of the new Byzantine empire. This was the beginning of the spread of Islam around the world as well as the rise of the Franks. 
Without the fall of the Western Roman Empire this would not have been possible. 

Answer 2

emperor justinian wanted to conquer what was lost when the empire collasped.he conquered lots of land and brougth the byzantine empire to its height