RootCuase: There is a problem with webparts...
Solution:
1. Open the site, for example http://SP2010/site/default.aspx
2. add "?contents=1" to the end of url, like this: http://SP2010/site/default.aspx?contents=1
3. You have a Web Part Page Maintenance and if you have some web part error delete it
4. Open the page with SharePoint Designer and all work correctly
