@peeld: It would be great if could give some more information.
- Please, open the widgets page of your blog in your browser
- open the Error Console of the browser (you can find it in a sub menu like Web Developer or Develop)
- clear the console
- reload the widgets page
If if your see errors then please report them here.
If you see no error message then try to change the position of the widget or add a different one and see whether the console shows errors now. If the error messages are long then consider using a pastebin service.
Please, describe what you have done to create the error.
It is also interesting know whether you are using a theme or a plugin which customizes the widgets page in a certain way?