I am working with Sitefinity 9.1 . I have created a user control named "Hello.ascx" in the folder "UserControl" within the root site folder. Registered this usercontrol to Sitefinity CMS backend within Toolboxes » Toolboxes » PageControls » Sections. But when I tried to add this widget to the page ,I am getting an error "the usercontrol cannot be resolved". Please do help me out to resolve this issue.
With 9.1, unless someone has a gun to your head, you should be using Feather MVC widgets :) Webforms ascx are bloaty and overly complex compared to MVC. Also an MVC widget will register itself in the toolbox.
Regarding this though, did you BUILD your project after you added the usercontrol? If you did build, can you paste in the corrospoding Hello line from your Toolboxes.config?
Either way though, thunder should be able to generate the stubs and register it for you.