Perhaps your database isn’t properly connected anymore and so you dont have any information flowing into Web Reports? You might double-check your web reports setting to make sure your database is not disabled (which will happen if it can’t connect for some reason).
I tracked it down to HKLM\SOFTWARE\BigFix\Enterprise Server\FillAggregateDB\RemoteDatabases\BES\Disable being set to 1 in the registry of our reports server.
Set it to 0, restarted the BigFix service - and up and running again.
I faced a similar issue before. Basically, the web report service tries to connect to the SQL before it is fully loaded. Solved it by editing the dependency for Web Report service to wait till SQL is started.