AWstats statistics data for previous year is erased on year change.
All AWstats data for previous year disappeared on year change.
This is known problem of AWstats if it is configured to produce static HTML reports (the mode which Plesk uses). In that mode AWstats produces detailed report for last month and less detailed report for a year. After new year date previous report is replaced with a new one. Thus previous year report is lost unless user made a backup.
Since 8.3 version Plesk stores previous year/month report before AWstats replaces it in folders like '/var/www/vhosts/DOMAIN/statistics/webstat/YYYY-MM'. So people upgraded to Plesk 8.3 before the New Year do not lose statistics for the previous year.