If you've added a poorly written webpart to your 'default.aspx' page, the consequence of this action will be that you won't be able to load the page anymore... Don't be sad, there is still a way to remove the webpart from the page :) (Yes, even removing the webpart from the webpart gallery would not solve this issue).
Steps to fix this:
1. Append the following query string to your URL: ?Contents=1.
Once you've added Contents=1 as a query string to the webpart page's URL it will display the Webpart Maintenance Page.
2. On that page you can now see the webparts added to the page, delete the problematic webpart.
Now try reloading the default.aspx page... Tadaaa!!! you can view your page again :)
3. Leave a thank you note @ comments section :)