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. Use FTP or PhpMyAdmin and rename the theme you are using. That will force wp to open with the default theme.
If the above does not help, delete the wp-includes and wp-admin folders. DO NOT delete the wp-content folder. Download a fresh copy of WP. Unzip those two folders and ftp them to the server.