It would certainly be surprising if soda was able to glue her head to that kind of strength, even with the sugar in it.
The others have all already stated that this pony's head isn't supposed to move, and it does look like it's in the correct position for this pose, but I want to humor you here, just in case. There is always a possibility, if you got her second-hand, that she'd had her head removed before for cleaning. You would know more about this particular pony's history than we do.
When you got her, do you explicitly remember attempting to turn her head? When you accidentally spilled soda on her, do you remember attempting to clean her then, or how you did so? Is her neck stuck completely all the way around, or are there any loose areas? Cleaning off something like soda should be as simple as using soap and warm water.