If you had average temperatures and a very sudden drop in temperature for a short period, that might cause this. Otherwise, it looks almost as if something's sat on the top of it, but all you can do is wait - check the mainstem, looking for soft or soggy areas, see if there's any damage there, but even if there is, its best to wait till spring (if you're in the Northern hemisphere) before cutting it down - if the roots are okay, it'll regrow.

We have had unusually cold temps the past few nights, here in Arizona. If you were in the path of the same cold front, it could easily have frozen, especially if it was planted in a low area of the yard, or other cold microclimate.

Oh, not you again, Mick1961, I seem to recall you joined once before just to add a few really tedious-but-intended-to-be-hilarious comments. Well, maybe you're laughing, no idea...yawn....

I have flagged both of his recent comments ... not helpful and not as funny as some Christmas cracker jokes.

Thank you for your responses.

We did just have a sudden cold spot here, and it wilted right after. I checked the stem and it seems solid and firm. There are just some plants I just don't seem to be lucky with.

You're probably just learning the hard way, like I did. :)
Beyond that, the weather here in the West has been terribly variable, the past couple of decades. Things that used to be hardy keep dying on us, both from excessive heat, and excessive cold.