I occasionally get e-mails from people who want to know what to do for plants that have been accidentally exposed to too-cold temperatures. Maybe they've picked up a plant that was abandoned next to a dumpster in a snowstorm, maybe they forgot to bring it in off the porch in the fall, that sort of thing. And it's occurred to me that I didn't already have a post about this, so maybe it would be worth writing one, since whenever I'm asked I wind up having to write a whole big thing from scratch.
The problem is, there's very little to say about this situation. There is no special fertilizer you can apply to reverse cold damage, and keeping the plant extra-warm for a while isn't going to help either. You pretty much just have to give the plant whatever care it would ordinarily prefer (or maybe slightly drier than it would usually prefer, if you think there may be root damage) and wait to see what it does. Sometimes plants will surprise you: I had a jade plant (Crassula ovata) once that was accidentally left outside, in Iowa, through I think mid- to late December. It was covered in snow when I saw it and brought it in, but it recovered and went on for several more years anyway.
Even in situations where it's clear the plant has been badly damaged, cold usually harms the extremities -- the tips of branches, the topmost leaves -- most severely. So, one can often just cut branches back to undamaged tissue and the plant will sprout new growth. (The resultant plant won't be as big and pretty as it was, but given enough time, it can come back.) Also anything that's obviously dead (black, crispy, mushy) can be cut off: whether it's an aesthetic improvement or not, those parts weren't going to spring back to life anyway, so you might as well.
Aside from those few things, though, mostly you just have to wait and see how bad it's going to be. And, obviously, not do it again. Within two or three months (probably sooner), whatever is going to happen will have happened, and you can decide what to do from there.