It has been documented that traffic can change for a certain website or app while reviewing analytics report. It is difficult to prepare an analysis as most often it is not clear from where to start the diagnosis. There is no best way to start the analytical reporting, the key being finding the exact reason behind the traffic drop; whether technical or marketing influence.  Traffic declines can be categorized into two category i.e. sharp declines or gradual decreases. Sharp declines over short stretches can point to technical issues while gradual declines may be inclined towards marketing and digital influences. Various solutions are available for change in traffic due to technical issues. To know more, follow:

http://www.cmswire.com/cms/analytics/where-did-it-go-understanding-traffic-drops-in-google-analytics-028468.php#null/