1. deactivating 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(s).
2. resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems.
3. switching to the Twenty Eleven theme to rule out any theme-specific problems.
4. re-uploading the wp-admin and wp-includes folders from a fresh download of WordPress.