Magento - page not showing up after upgrade or install

Check your permissions. This is almost always a permissions problem. Sometimes you will get a 401 and sometimes a 505 error. But check your permissions on the folders and the index files based on where you are finding a problem. Folders are usually good with 755 permissions and files with 644.

The following need to be writable:

magento/var/.htaccess
magento/app/etc
magento/var
magento/media/import