I am writing this issue, and it's resolution, to avoid anyone the similar trouble I went through to solve it.

I had a running website, that I was developing on a PC running Windows 10. I had to re-install Windows 10, and this is when the problem occurred:

On my existing project, I had the error (on IE11)

Can’t reach this page
•Make sure the web address http://nyg.debug2 is correct etc

So no tangible pointer to start with.
So I created a new project, and it run fine with IIS Express.
When trying to run it with IIS, I got the error:

HTTP Error 500.19 - Internal Server Error
Error Code 0x80070021

This was an indication that ASP.NET 4.5 was not installed.
And as we need ASP.NET 4.5 to run Umbraco (as I learned), you have to install it.

For more info on how to install ASP.NET on Windows 10, I refer to http://www.jammer.biz/enable-asp-net-on-windows-10/

Basically what you need to do is; 1) Open PowerShell as Administrator 2) Run

dism /online /enable-feature /all /featurename:IIS-ASPNET45

Then both my Umbraco sites (my existing and my new one) where up and running.