I had a similar situation with a microwave - I bought it, and a couple months later it stopped heating, and the service center immediately claimed I was using it wrong somehow. Turned out it was actually a factory defect, but the store didn't want to deal with it and just kept insisting it was my mistake.
Here's the thing they didn't tell you (or hid from you) - the store can't just refuse without an official diagnostic. If the device won't turn on at all and you didn't drop it or get it wet, then it's clearly not user damage. Demand a written refusal - when you ask for something in writing, sellers suddenly get serious. Ask them to run a diagnostic and put the results on paper with the store's stamp. If they refuse - that's already a breach of their obligations.
Rospotrebnadzor will help, but try filing a complaint first. Write a complaint in two copies (one for them, one for you with a received stamp), copy your receipt and warranty card, cite articles from the consumer protection law. If the store digs in - then yeah, go to Rospotrebnadzor or even consider filing a court claim. Usually by that point the store suddenly remembers about warranty repairs.