According to the official support thread, you can:
- Check out if you have the latest version downloaded and installed on your website. Head over to
My Account -> Downloads on your WPML account.
- If you need help updating, you can check this out.
- If none of the above helped you, consider submitting a support ticket on the WPML support forums.
Now that we are off the official guidelines, we can stick to our own methods.
Try disabling the WPML plugin via ftp, or fully move the folder to somewhere else just to see if you can login to back-end.
If you can't use ftp, you can use WordPress command lines to deactivate the plugin.
After that you can login to back-end and install or update the WPML plugin to resolve the issue.