I edited the /etc/rc.local to run a python script. It works, but it never ends, I tried Ctrl+c but it does not work I can't get in to the log in / password display.
Try ctrl+alt+f1 to get a console window (ctrl+alt+f7 to return).
You can login and edit/kill the script and reboot.
Search for password recovery.
Alternatively edit cmdline.txt (it's on a FAT partition so is visible on a Windows machine) and add init=/bin/sh to the end of the (single) line. Reboot and edit rc.local then undo the change to cmdline.txt and reboot again.