IIS error - Unable to display page or site - .NET framework error is displayed

If you are using IIS with .Net framework web site you need to install .Net after IIS.

If you install IIS after .Net framework you need to run:

C:\WINDOWS\Microsoft.NET\Framework\".net version"\aspnet_regiis.exe

If it doesn't work you can run it with the following parameters
-ua > to uninstall all versions
-i > to install that specific version

Other Parameters:
-i - Install this version of ASP.NET and update scriptmaps
at the IIS metabase root and for all scriptmaps below
the root. Existing scriptmaps of lower version are
upgraded to this version.
-ir - Install this version of ASP.NET, register only. Do
not update scriptmaps in IIS.
-enable - When -enable is specified with -i or -ir, ASP.NET will also
be enabled in the IIS security console (IIS 6.0 or later).
-s - Install scriptmaps for this version at the specified path,
recursively. Existing scriptmaps of lower version are
upgraded to this version.
E.g. aspnet_regiis.exe -s W3SVC/1/ROOT/SampleApp1
-sn - Install scriptmaps for this version at the specified path,
non-recursively. Existing scriptmaps of lower version are
upgraded to this version.
-r - Install scriptmaps for this version at the IIS metabase
root and for all scriptmaps below the root. All existing
scriptmaps are changed to this version, regardless of
current version.
-u - Uninstall this version of ASP.NET. Existing scriptmaps
to this version are remapped to highest remaining
version of ASP.NET installed on the machine.
-ua - Uninstall all versions of ASP.NET on the machine
-k - Remove all scriptmaps to any version of ASP.NET from the
specified path, recursively.
E.g. aspnet_regiis.exe -k W3SVC/1/ROOT/SampleApp1
-kn - Remove all scriptmaps to any version ASP.NET from the
specified path, non-recursively.
-lv - List all versions of ASP.NET that are installed on the
machine, with status and installation path.
Status: Valid[ (Root)]|Invalid
-lk - List all the path of all IIS metabase keys where ASP.NET is
scriptmapped, together with the version. Keys that inherit
ASP.NET scriptmaps from a parent key will not be displayed.
-c - Install the client side scripts for this version to the
aspnet_client subdirectory of each IIS site directory.
-e - Remove the client side scripts for this version from the
aspnet_client subdirectory of each IIS site directory.
-ea - Remove the client side scripts for all versions from the
aspnet_client subdirectory of each IIS site directory.
-? - Print this help text.