Rescue mode will boot without attempting to import the pool. On the latest versions you will see "Rescue Mode" as an option. On older builds you will follow the below steps.
Booting into rescue mode: You'll reset the server and when you get to this screen. After the version is selected a second screen comes up (see second photo). If you see "Rescue Mode" select it. If not continue on to hit TAB.
Use the arrow keys to select "Launch system in RESCUE MODE (skip import pools)", if that option is not there hit "tab" and keep following the instructions below.
On this screen press "end" on keyboard, then "space", and then - at the end of the command line add “rescue_mode” and press ENTER.
Let the system boot up and when it comes up we will import the pool.