Why my phone switches on and after switches off?

Why my phone switches on and after switches off?

My cell phone was flat when I try to charge it, It wasn't. I tried to change the battery to another phone and the battery was charging. I took it for repair they said its a software problem but thay cant fix it because they the phone cannot ready the software. now I want know it means now my phone is useless? there is no way it can be fixed I mean its 1year old.