First of all, don't worry about it so much. The .NET ecosystem is very complex and I find the barrier to entry for any .NET language to be far higher than that for the likes of PHP. If you've jumped straight into a .NET role without any prior experience I'd be surprised if you didn't feel overwhelmed when dealing with a full website.
However, I'm not going to lie to you in that the task you've inherited is a massive one, and that it's going to take you at least a couple of weeks of solid reading and playing with the code to have any idea of what's going on. You'll need to pick up a basic ASP.NET book tailored to working with VB.NET. Luckily, SitePoint offers a book that is tailored towards the beginning programmer with code in both C# and VB.NET, titled Build Your Own ASP.NET 3.5 Web Site.
Your first port of call is to your employer to state that you're naturally not a programmer and that for even experienced coders it would take a while to get used to the code. Secondly, you'll have to put in the time to get yourself up to scratch with the code itself. If I were you I would take a two week period to read the book I mentioned (or similar) to acquaint yourself with the stuff you'll be using, and to then start toying with your code.
It may seem like a monstrous task, but if you've got the willpower and the interest to learn this stuff then it is possible for you to train yourself up enough to maintain it and work from there. It'll be hard work, but I'm sure you can do it.