Try the following
-deactivate all plugins to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin.
- switch to the Twenty Eleven theme to rule out any theme-specific problems.
- reset the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems.
- re-upload the wp-admin and wp-includes folders from a fresh download of WordPress.