You probably want to use a single user mode. The way to invoke it depends on your exact distro. Very often it consists of just adding some boot-time option ("1" or "single").
This can be done from your bootloader.
In Grub just highlight the kernel you want to use, press 'e', go down to line starting with 'linux' and add the boot-time option to the end of that line. Then press enter to boot that kernel with that option.
For Debian the way the process is described here. Hopefully it should be the same for Raspbian Jessie which is debian-based.