When upgrading from an older version of .Net to a new one you are going to need to set this into IIS7 so that it knows which engine to use when handling a website (it will not check this on the webconfig as, to start with, it doesn’t know there is a webconfig without knowing the engine to handle that website).
Inside IIS 7, notice you have a node with the server/PC under which you can find “Application Pools” and “Sites“. Under Sites you will have configured your websites, but under Application Pools you need to configure the engine each of them is using.
So we go to Application Pools > Choose the one your website is using > double click > change .Net Framework version > accept.
If you don’t know which Application Pool your website is using simply select it and, on the right sidebar with options select “Basic Settings..” (5th option from top). You can see the Application Pool there.
