Locked out of my site!!! Help!

*it was a plugin

Just delete whatever it is. If its a just a php file, a theme or a plugin. Delete it (keep a copy on your pc first).

how, if I can’t get into my site. Through the FTP (i think its called that..?)

Yes ?? File Transfer Protocol. Download a FTP program if you already don’t have one, FileZilla is the classic. Or by using CPanel, if else your control panel your host offers.