i want to restart my career in developement but in which

That really depends on your interest considering that there are bunch of development platforms to choose from. But since you are coming from a Microsoft dev stack background, then I would recommend you to start looking into ASP.NET. ASP.NET provides a variety of platforms that you can use to get yourself into web development. ASP.NET offers a few frameworks for creating web applications: ASP.NET Core, ASP.NET MVC, Razor Pages, Blazor and Web APIs. All of them frameworks are stable and mature, and you can create great web applications with any of them. No matter what framework you choose, you will get all the benefits and features of ASP.NET everywhere.

Now which web development framework to choose?

Since you are coming from a Windows Forms development background, then you can start jumping into WebForms to take advantage of the design surface, powerful server controls and components. This is the easiest entry point to ASP.NET because you are already familiar with the drag-and-drop and event driven model from windows forms.

If you are willing to take the challenge, I would really recommend you to learn ASP.NET MVC over WebForms.

Why?

(1) Provides a clean separation of concerns (the Model, the View and the Controller)

(2) Gives you full control over your codes, HTTP, Request, Response, JavaScripts, CSS, HTML and so on

(3) Provide clean and friendly Urls through it's built-in routing capability

(4) You'll get the feel of real web development because there's no abstraction involved.

(5) It's easy for you to learn Web API as it follows/uses Controller based pattern like MVC

Alternatives: You could also try learning SPA technology like Blazor, Angular or Knockout as front-end, but this would probably give you a big learning curves as you will mostly dealing with client-side scripting. But if you really want to be a real web developer, then you have no choice but learn these technologies. Smile | :)

But, Where do I start?

You can pick a book that you are interested or refer to online tutorials and apply three things "READ, DO and LEARN. Learning something is a just matter of doing something.In my own experience, the easiest way to learn is to develop a sample application in your own by applying the things you've learned from the book or tutorials. I'm sure you will learn a lot as you go along implementing your own web application.

HTH! :)