Now I can't boot

There have been plenty of times by now that I have made some sort of adjustment on my system and then it doesn't boot.  We know the usual suspects are /etc/rc.conf and /boot/loader.conf but I'm sure there are others, possibly even a badly thought out recently built and installed custom kernel.  So now we are stuck, we have one box and it fails to boot but the way to solve the problem is to get online from it.  The situation with the broken kernel might be sidestepped easily, simply choose the option from the boot menu to use a different kernel.  If there were also mistakes with the buildworld, and lack of items means no booting, then there needs to be another way.

If you can get to single-user mode (another boot menu item), the changes will be easy to apply.  First, mount -u / and then if your filesystem is ZFS rather than UFS, zfs mount -a and now you can re-edit the typo out of your /boot/loader.conf or some other file, but what if your situation is a bit more complex?

You can use a usb stick to boot and from there mount the drives in your pc, make needed adjustments and get everything back to normal again.  This is where it can be fun, and when I say fun I mean not quite a nightmare though it is a real special pain.  You can probably use any bootable BSD which offers shell access to the machine, but since discovering NomadBSD, it has become my preference.  What NomadBSD has is a complete system which is self-contained within the usb media.  So if you are short of time and cannot fix your system, you can use it to get online to do something important, such as check your work schedule.  Of course, this immediate need situation means that you previously setup a web browser and installed and configured an addon (such as blur by Abine) which stores your passwords, and you had made any other needed adjustments to suit your needs.  So, getting online you have the solution to the problem and you've written it down and now you need to fix whatever is wrong on the HDD of your system.

What you need to do is mount the HDD of your system into the usb media that is loaded.  There should be a directory /media present already, if not, create it via mkdir /media because this will be the mount point to reach inside your system HDD.  I would assume that you are already in a shell window (xterm perhaps) or you used shell access from the menu when you booted the usb.  We have two pieces of the puzzle, the solution and the system running with a shell, what we need to do in order to make the changes is get to the drive.

With ZFS, there is a special command which will do what we need, change zroot to the name of your pool.
zpool import -f -R /media zroot

Many times the mistake was made or is corrected in either /etc or /boot, so now to reach those directories or any others on your HDD, you would prefix the desired directory with /media such as below.  Work slowly, re-read the command you've typed before committing to it by pressing return or enter.  While your HDD is mounted to your usb stick, /media is the root directory of your HDD, and / is the root directory of the usb stick itself.  Entering the specific directory in order to edit a file, such as rc.conf or loader.conf, may be better than remembering every time to prefix with /media, but always pay attention to your current working directory or path.
cd /media/etc
or
cd /media/boot

Only you can know what the problem and solution are.  Now that you have access  to your HDD you can make the corrections and reboot.  That zpool import command is only viable until you reboot and does not need to be turned off or disabled.  We have not made any permanent changes to how your drives are mounted in order to fix the problem, unless of course your problem and solution specifically involves a permanent adjustment to how your drives are mounted.  Right now I do not have any examples handy of the dumb things I have done which resulted in being unable to boot along with how they were fixed.

While tinkering with your system in ways that can only truly be done when it is open source, because you use FreeBSD for your need of control over all of it, you are setting yourself up with the potential for mistakes.  There is nothing wrong with unintentionally doing something incorrectly, it is the surest way to learn.  We may read somewhere how to do something but unknowingly miss a step or configure something wrong or assume their technique will work on our system.  The worst of these experiences involve Boot blockers and unfortunately they can at least temporarily halt all further progress.