[Solved] ASP.Net Error: The TextBox element is not known. This can occur if there is a compilation error on the site or if the web.config file is missing

Hello, I'm developing a web application in Visual Studio 2019 and, suddenly, all the components are no longer recognized. Check the web.config, I even copied one from another application and it still didn't solve the problem. I've also seen the references, all of which are correct. The message returned is:

The "TextBox" element is not known. This can occur if there is a compilation error on the site or if the web.config file is missing.

How can I solve? Grateful, Ilano.