Constant charging / sync / startup sound but no display no boot
Hi, I've purchased a iPhone 4S off ebay with the aim of fixing it. It is in great condition inside and out, however won't turn on or boot and there is no display. When plugged into the wall socket it just makes the charging chirp sound over and over again, but that is all. When plugged into a computer it does nothing, no show in iTunes. All the seller was able to say was that he tried to fix the screen and it worked before that, but couldn't get it to work properly after that. Any ideas what could be wrong, what needs to be replaced? If it was just a faulty screen shouldn't the phone still charge and why is it making the constant sound?