There are a lot of things this could be. Using FTP or by signing into your hosted account open the wp-config.php and your theme’s functions.php. Make sure there is no extra white space or line before <?php
Next, was this an upgrade? Have you tried:
– deactivating all plugins (yes, all) to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s). Don’t forget the ones in mu-plugins. If you can’t get into your admin dashboard, try resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems.
– switching to the Twenty Eleven theme to rule out any theme-specific problems.