You should check the System, Application and Solarwinds.Net event logs.
The log file \Program Files\Solarwinds\Orion\NetFlowTrafficAnalysis\swdebug.log(.x) should give you the exact error as well.
I checked those logs and didnt find anything for the time/date stamp. Had the system admin reboot the box. It started collecting again. Then today 5-27, I noticed that it stopped collecting again on 5-21 at approx 6:00pm. Reviewed the logs in the event viewer and the files in the directory that you suggested and none of them had a date/time stamp for 5/21 at 6:00pm.
What could be causing my Netflow collectors to stop collecting data? Obviously I dont use them every day but I am collecting for my boss and dread the day that he questions me and I have no data for him.