HTTP ERROR 500

Hi,
When trying to update our logo on the website, ERROR HTTP 500 has occurred. This occured after i uploaded a plugin to assist with the upload and adding the plug in code to the header.php. The site now appears to be down and i am unable to sign into the admin. I have downloaded cyberduck and another program to manually deactivate the plugins but I just can’t seem to work it out. Please any help is appreciated.