If the power button is stuck, it might be due to pressing the button at the wrong angle,constant usage wear and tear, or a foreign body causing the sticking. If it is genuinely will not function correctly, then request a replacement under the warranty (if valid!). I have never had a stuck on/off button, but then I never use them, only the mains switch off for all the systems.
Warranties can and do vary. In the main it is usually a 3 year warranty, which can be a 'swap-out' or RTB. Some warranties can also be a 1 year cover (which the retailer would responsible for any way). Usually the lesser warranty is due to an agreement with the retailer and a possible extended warranty arrangement set up by the retailer. You do not mention how long you have had and used the unit!.
Perhaps as a future reference, any purchases of a similar nature, made in the future, retain all the information via printing out, then you cannot go wrong at a later date!.