If you've deleted the DLL or if you reinstalled a DLL which no longer contains the codebehind class for a user control in a web part, you could end up in a scenario where you have a "dead" web part on the page. In my case, it prevented the page from loading and resulted in an error because it couldn't load the type.
Don't panic! The solution is to add a ?Contents=1 to the URL (i.e. http://moss.dev.com/default.aspx?Contents=1), which will allow you to delete the web part from the page.