Crashing Moz rank in one month 4.8 to 2.8??

My Moz rank dropped from 4.85 to 2.8 in one month  June and since then it has remained at 2.8 - cant really find any reason for this - any advice or suggestions? It was consistently 4.8 or so for around 18 months so it is peculiar

thanks 

Sujoy