Have you tried:
- deactivating ALL plugins temporarily to see if this resolves the problem. If this works, re-activate them individually (one-by-one) to find the problematic plugin(s).
- switching to the unedited default, core-bundled Theme (such as Twenty Twelve, Twenty Thirteen or Twenty Fourteen) for a moment using the WP dashboard to rule out any theme-specific issue.
- resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems (because the hooks remain unless plugins completely removed or some plugins stick around in cached files. So by renaming the folder, you break them and force them inactive).
- If the above troubleshooting steps fail to resolve the issue, try MANUALLY re-uploading all files and folders EXCEPT the wp-config.php file and the /wp-content/ directory from a fresh download of WordPress. Make sure that you delete the old copies of files & folder before uploading the new ones. Read the Manual Update directions first!
- BTW, if you haven't already done, always backup everything (including your database) before doing any actions, just in case something really goes wrong. It's a great safety net.