iPod touch died, no damage, iTunes doesn't recognizes it, black screen

My iPod touch 5th Gen has black screen. It was working good last time I used 3 weeks ago.

When a tried to switch it on , there is no answer, keep black screen. I tried also reset holding home button and sleep button and nothing.. tried all possible combinations of holding buttons and cable connected to computer and nothing…. In apple store told me I need to buy new one because mine is died. I’ve toke care of my ipod, any damage, any hit, any wet, it was in perfect shape, also the battery was in good conditions.

I bought a new battery and I change it but still same problem, iPod like a stone.

Any advice what should I test to find where is the problem. which steps I should follow to testing every component to localize the problem?

Thanks.