A bag of frozen peas or corn. Lay on it or massage it on your neck. When it starts to thaw just toss it back in the freezer. Make sure to mark it though so you don't eat it since the constant freeze and thaw can make it go bad and make you sick to eat it. Rotate every 30 minutes with that and a hot towel or heating pad and take ibuprofen to keep the pain at bay. Make sure not to do too much once the meds kick in. You won't feel as much pain at the time but when they wear off it'll hurt more from the overworking.
I've always had back and neck pain and this is what I was told to do on bad days.
do you know why you have a stiff neck?
could it be from working? working differently? sleeping funny? breastfeeding? stress? an accident?
is this a one time thing, or has this been a constant or recurring thing?