What you are seeing is the "white screen of disappointment" This can happen as a result of an error in a recently edited php file or an incompatibility in a recently installed or updated plugin or theme.
If you have recently installed a new plugin, connect to your files using an ftp client app on your computer such as filezilla and delete the plugin you have just installed.
If you have recently edited a theme or plugin file, connect to your site via ftp and reverse the last change you made.
If have a backup of your files taken before making the change, you can just replace the edited file with the pre-edit backup of that file.
If you know which file you edited but don't have a pre-edit backup and you are unsure of which lines were edited, you can download a fresh copy of the theme or plugin to your desktop, unzip the files then replace the file you edited with the new copy.