There are various causes possible. Please elaborate a bit more about your environment (physical server or VM, installation-type and number of server/nodes and so on) and how the upgrade was done (with de-installation or just a new installation, parallel installation, with the already used admin-account or another, any further changes, upgrade of all services or just the server+webserver)?
Thanks for your response,
As we have identified the issues. The issues was that we did not installed the Qlik Server component in web server (Access point) .We had just considered the web server while new qlik version installation in access point drive which used to client access point.
Also we had faced the DNS issues even all services was running however Access point url i.e www.anaXXXX.com was not active later on ,this issues resolved byIT team.