This morning when I login to one of none production environment I found that the graph was not loading on Sitecore CMS launch pad.
When I did into more details and found out the ajax request to get the data are failing with server error(HTTP-500). As shown in below image the response of the API call does not providing any information or clue.

Immediately, I jumped onto logs and found following entry in there.
The remote server returned an error: (403) Forbidden.
The full exception looks like below.
50616 09:48:50 ERROR [Experience Analytics]: System.Net.WebException: The remote server returned an error: (403) Forbidden. at System.Net.HttpWebRequest.GetResponse() at Sitecore.Xdb.Reporting.Datasources.Remote.RemoteReportDataSourceProxy.GetData(ReportDataQuery query) at Sitecore.Xdb.Reporting.ReportDataProvider.c__DisplayClass18_0.b__1() at Sitecore.Xdb.Reporting.ReportDataProvider.GetCachedDataForQuery(ReportDataQuery query, Func`1 ifNotAvailableInCache, Nullable`1 ifModifiedSince) at Sitecore.Xdb.Reporting.ReportDataProvider.ExecuteQueryWithCache(ReportDataQuery query, ReportDataSource dataSource, CachingPolicy cachingPolicy) at Sitecore.Xdb.Reporting.ReportDataProvider.GetData(String dataSourceName, ReportDataQuery query, CachingPolicy cachingPolicy) at Sitecore.ExperienceAnalytics.Api.ReportDataService.ExecuteQuery(IReportQueryData queryData, CachingPolicy cachingPolicy) at Sitecore.ExperienceAnalytics.Api.ReportingService.RunQuery(ReportQuery reportQuery) at Sitecore.ExperienceAnalytics.Api.Http.AnalyticsDataController.Get(ReportQuery reportQuery)
Fix
After further investigation found out the Reporting keys were not aligned between CM and Reporting server in the ConnectionStrings.config file.
<add name="reporting.apikey" connectionString="84090dc97055b10532a054da067df368" />