Question: Do yeast die after bottling? 

My second batch of beer have been stored in bottles over a month now without producing any co2.
I want to add some priming sugar to start the fermentation again, but I'm unsure if it will help.

Answer: 
Yeast will become dormant and eventually die after a few weeks to months, but only after any food sources, like priming sugar, have been consumed. 

The lack of carbonation after a month could be caused by a number of things

* insufficient yeast - normally there is yeast in suspension after primary, but a highly flocculative strain may settle out completely. For bottling, 10,000 cells per ml is considered a typical cell density.
* if the priming sugar was added to the entire batch, it may not have been mixed with the beer fully, leaving some bottles without sugar and others an excess.
* the bottle caps are not on airtight
* your temperature of 19-25c should be warm enough, but typically cold temperatures can hinder fermentation.

Adding more priming sugar can be done on a bottle or two as a test, which you can open after 3 days. You should hear a hiss, but the beer itself won't be carbonated unless it's left for another week. But if you get the hiss then the yeast are working it's most likely a problem with distributing the priming sugar evenly. No hiss, and you may need to pitch some more yeast. A tiny pinch of yeast is enough for the entire batch, but putting a tiny pinch of yeast in each bottle will work also.
I have just revived a liquid yeast that was in the fridge for 2.5 years, I cleaned it up, did a starter and its like new.. I took some microscope samples and didn't see any bacteria infection or mutations and even made a little fun video on it, for anyone interested...

[co2 gas bubble moving under the slide, live sample from active fermentation](https://www.youtube.com/watch?v=UQQQuAF74Ig)