Accidentally swallowed mouthwash! Help?

You'll be OK. If it was just a mouth-full it won't affect you much at all. If you are drinking it however, that is another story. Please don't do that.
What & how much? If you just swallowed a cap full or so, it is probably ok. If you get nauseous or feel ill, seek medical attention immediately. It also depends on the type and strength of the mouthwash.