IIS not working after upgrade to Windows 10 Pro from Windows 7 Unlimited

Since the accepted answer requires IIS Manager, and IIS Express doesn't have IIS Manager or any UI, here's the solution for you IIS Express users (and should work for everyone else too):

When you open Visual Studio and get the error message, right-click the project Solution Explorer and choose "Edit {ProjectName}.csproj"

In the project file, change the following line:
<UseIIS>True</UseIIS>
to
<UseIIS>False</UseIIS>
Save the file.

Now reload your project.
Done.

You'll then be able to open your project. If at this point, you want to use IIS, simply go to your project properties, click the "Web" tab, and select the option to use IIS. There's the button there to "Create Virtual Directory". It may tell you that you need to run Visual Studio as an administrator to create that directory, so do that if needed.

It could be that the user identity is outdated, especially if you've tried starting a stopped app pool and the next request again fails.

In IIS, go to the Application Pools under the Server, then find the correct Application Pool for your web site, and click on it. On the Advanced Settings menu to the right, select Identity and change it and enter new user and password. Click on your Application Pool again, and select Recycle to restart it.

You can also try looking at the error message in Event Viewer, under Windows Logs, Application, Details tab.