By 'What happened', what do you mean. Was it removed? According to the Wikipedia article, a new NBC station near Watertown signed on about a year ago, so WPTZ could have been dropped due to market area rules if you are closer to them.
What happened to WPTZ channel 13???
WBTS-TV from Boston is the local NBC station assigned to your area. WBTS can be found on channel 22 and in HD on channel 1010.
Boston is no way on earth near my "local" area. I dropped Dish Network because of this stupidity. It's pretty sad that I have to shut off cable and hook up digital rabbit ears to see news and weather that actually is in my area. Makes me question why I bother with cable at all :-/
Apologies. From your location, it is less than 100 miles to Boston, MA (where WBTS is licensed) and over 185 miles to Plattsburgh, NY (where WPTZ is licensed).
All of Cheshire County, NH is 100% in the Boston-Manchester television market as determined by the FCC and Nielsen Media.
Map linked below shows the US counties by TV market.
WPTZ-TV is considered an "out-of-market" station in this area.
The terms "DMA", "out of market" and "significantly viewed" all have extensive definitions and examples, some links below that will make for some good reading:
Out of market and Significantly Viewed:
You can certainly contact the FCC about any questions on the "significantly viewed" definition or contact Nielsen regarding the current DMA designation for your area.
If it is important for you to view WPTZ, you may be able to receive this station over the air with an antenna depending on your location and surrounding terrain.
As you already ran into, other video providers are also required to adhere to these television market zones.