You implement your web part, build and deploy it to your SharePoint server. However your web part page can give errors due to a bug in your code or some other problem. Unfortunately in most cases web part page can not be opened in edit mode. There are many scenarios that causes this kind of problem.
In this case you have two options. First one (and unnecessarily time-consuming) is removing web part from web part page by SharePoint Designer.
The better solution is using Web Part Maintenance Page. This is a very simple way to manage web parts in you web part page.
Just add ?contents=1 to the end of the URL of your web part page.
Web part maintenance page can also be used to access the closed web parts in your web part page.
Note: You cannot use the Web Part Maintenance Page to maintain a web part outside a web part zone. In that case you have to use SharePoint Designer option.