Cannot modify header warning

I’m now suddenly getting this warning and can’t login to the admin backend anymore. Read NUMEROUS posts on this forum about this warning and simply cannot figure this thing out. I’ve checked the files for whitespace, in this case it’s complaining about my_functions.php on line 1 and pluggable.php.

My site has been working for months and I just received this warning today. The only edit I made was to the end of my_functions.php definitely not line 1. And then I tried deleting a plugin and when it asked if I wanted to delete all files and I clicked yes, this warning appeared and i couldn’t delete all the files. I then looked for the files on via FTP and none of them were there. Any help? Please!