If you have a white page when you go to your Joomla site or when you go to or login at the administration page, try going into the plugins, modules and components folders in your document root and rename the 3rd party plugins and modules that you have installed and many times it will fix the issue. This is usually caused by plugins that are not compatible with the version of Joomla that you are using. If you rename them and you are able to log into the site, you can then go back and re-enable them one by one to find the exact problem addon.