Looks like a “white screen of PHP death” – basically something, somewhere in your WordPress install, some PHP code got messed up to the point that when the PHP interpreter on the server runs the code it errors out, thus producing a white screen. Check your server error logs to see if they provide clues about what might be messing up.
How to get a functioning WordPress site again: Make a backup of all your WordPress files on the server and save to your local machine, then download the latest version of WordPress, unzip it, and then upload the new WordPress files to your server overwriting the existing files – excepting the uploads folder and the wp-config.php file (DO NOT OVERWRITE THESE!). You should then have a functional WordPress site although you will have lost any custom themes and all your plugins.
Now carefully re-introduce your custom theme and see if it works – if not, the issue is in your theme and you should contact the theme developer for support. If your custom theme works, start adding in plugins, one at a time and enable them. Test the site after you enable a plugin. If everything works, move on to the next plugin. If things go bad, disable/delete that plugin, make a note of which one it is, and contact the plugin developer for support. In this way you should be able to track down what is behind your issues.
