I don't want to frighten you, but honestly, I would have an xray done before next week. There is a condition called Atlanto Axial Subluxation that can show up in yorkies...we have seen a few on YT this past year. They suddenly develop neck pain with that condition.
Here is a link with information:
ACVS - Atlantoaxial Instability
While it very well might not be that, it is something I would want ruled out immediately if I had a pup with sudden neck discomfort.