I have resolved the problem , I had a bkup of my config.xml file from C:\ProgramData\QlikTech\QvWebServer. I replaced this file & found that in <AccessPoint> tag
<AddQvs> <Machine>172.28.64.8</Machine> <Port>4747</Port> <LinkMachineName>gdac.myigetit.com</LinkMachineName> <AlwaysTunnel>False</AlwaysTunnel> <Weight>1</Weight> <Username /> <Password /> </AddQvs>
was missing . But in my QEMC UserName & Password was blank but in Config file this tag's were missing. I replaced this file & it is working now.