You probably have a plugin and/or theme conflict. Did you just update a plugin or theme before this happened?
=> If so you'll have to FTP into your site and navigate to the wp-content folder. Go to the plugins folder and find the plugin you just updated and delete or rename that plugins' folder (I tack on and ORIG on the end). Then check to see if you can get in.
=> If not, rename the entire plugin folder and try again. Renaming the folder will simply deactivate the plugins so that at least you can get back into to your dashboard. Once you are in you can rename the folder to the original names (pluginsORIG back to to plugins) and then reactivate one-by-one to see which is the problem child.
=> If you are still stuck, go to the themes directory and find your theme's folder and rename it.
You can find out more here. HTH!