If you add near boiling water to fermenting wort, then yes, you can definitely kill some of the yeast, at least, any yeast that come in contact with that near boiling water. If there was enough yeast in the fermenter, distributed in other parts of the beer, then a lot of it may still be alive. If you see signs of fermentation (bubbling airlock, krausen) it worked. If you don't, it still may be working but you'll need to check the gravity a few times to know if it's woken back up.
In the future, you should not do this again :) You can warm it up by other means, and generally, unless it has gotten very cold, you can wake the yeast up again. You can
- move it to a warmer spot (next to a heater/radiator)
- use a brew belt or electric heating pad to warm it up slowly. You can add insulation, blankets, coats, etc to keep heat in.
- put it in a sunny spot, completely covered in something dark to block out the light and absorb sunlight/heat. Blocking the sun is important to avoid skunking the beer, but you can still use the sun to heat it
- put the fermenter in a warm water bath
With all these methods, the temp needs to be monitored so you don't overheat the yeast. Once it's warmed up, you can also add some more DME, sugar or honey in a solution of boiled water, cooled down first to help activate the yeast, or add more yeast.