Try disabling compilation to start if the compilation was not disabled before upgrade the compiled files may have become out of sync with the Magento Version you are now currently on.
1. Disable Compilation
php -f shell/compiler.php -- disable
Run above command from root magento directory if you have ssh access.
Rename the folder to includesbak or comment out the 2 lines within includes/config.php which should also disable compilation.
2. Clear Cache
rm -rf var/cache
Run above command from magento root or delete folder with ftp.
3. Check errors
Add below to index.php to show errors this may further help us rather than just guessing.
You can also check var/log/system.log and other log files within var/log folder for errors. Check what errors show up when you visit admin. If none of the above fix the issue then add some errors to your question found in these logs and we may have more hope figuring the real cause.
I would also recommend atleast moving that admin url to something custom to your store.