Markus was completely right, you need to 'rebuild' your sitefinity.dll (local or remote).
By default when you create a .ascx file you'll have a top line something like this:
<%@ Control Language="C#" AutoEventWireup="true" CodeBehind="uc_widget.ascx.cs" Inherits="SitefinityWebApp.widget" %>
" means you have to rebuild your project every time you make changes to your widget and upload the widget and the .dll.
If you change it to "CodeFile
" you just have to copy the widget files (including the .cs) to the server.