Cannot start Power BI Report Server after upgrade (August 2018)

Hi , I'm  facing same kind of issue .i'm not able to Start my report server in service it says "window cannot start powerbi report server". yesterday my password got expired i tried to start with new credentials in the Report server configuration manager but it says "task failed to start power bi report server ". Since this is production server with many users so  i don't prefer to uninstall.  kindly assist to solve.

-- logfile -----------------

2020-06-13 16:25:54.1225|INFO|1|File Logger created: C:\Program Files\Microsoft Power BI Report Server\PBIRS\LogFiles\RSHostingService_2020_06_13_16_25_54.log - level Info, will roll at 32 Mb, process id 7380
2020-06-13 16:25:54.1537|INFO|1|Current Directory : C:\Program Files\Microsoft Power BI Report Server\PBIRS\RSHostingService
2020-06-13 16:25:54.4819|ERROR|1|Exception occurred while parsing config file at [C:\Program Files\Microsoft Power BI Report Server\PBIRS\RSHostingService\config.json]System.Xml.XmlException: Root element is missing.
at System.Xml.XmlTextReaderImpl.Throw(Exception e)
at System.Xml.XmlTextReaderImpl.ParseDocumentContent()
at System.Xml.XmlReader.MoveToContent()
at System.Xml.Linq.XElement.Load(XmlReader reader, LoadOptions options)
at System.Xml.Linq.XElement.Load(String uri, LoadOptions options)
at Microsoft.BIServer.Configuration.ConfigReader..ctor(String configFileFullPath)
at Microsoft.BIServer.BIService.ServiceConfig..ctor(String serviceConfigFilePath)

Thanks in advance