Electromann accusing us of opening and tempering with an item because they don't want to replace or refund it

I purchased a power supply. The item was not working,I sent the item back and it was sent back to the supplier. A month later I got feedback from them saying that the warranty on the item falls away because the item was opened up and tempered with. I take insult to that because it was not opened up and tempered with. This is absolutely ridiculous. someone supposed to call me yesterday to get back to me about where we are at but no one ever calls back.