So I happened to do the cardinal sin of administration. I changed the openssh settings, and accidently closed that terminal before I could test it, and of course, now I'm completely locked out. I was using password-less private key authentication. Am I completely screwed out of my server? This is a VPS system -- is there a way where I could have someone at the datacenter log in locally and redo the ssh settings?
If you lost your SSH key, it's gonna be quite complicated. I don't really know if somebody can log in your VPS, but that means you would have to provide your root password. Do you have a backup? I know that some VPS allow to log in via console, Linode provides it, for instance. I guess at least you might talk with your provider.
In any case, you should learn about this, we improve our skills due to our failures.
This is where your DC remote hands service comes into play. What level of shell support they provide for you varies with your colo and service agreement.
I'd provide a printed set of specific commands to run to restore settings to your preferred condition, along with any relevant files (e.g.: the correct SSH key to copy over/into the user's authorized_keys file).
Otherwise, plan on paying your server a personal visit.