This has worked for me numerous times:http://thinkinginpixels.com/quick-fixes/fi...onlog-off-loop/
However, after you are able to log in correctly you should perform necessary malware scans with a quality commercial malware scanner.
Also scan with Malwarebytes anti-malware.
Be sure to update all your scanners before performing a scan.
After you reboot the problem may come back if it is not resolved and you will need to run the fix again.