My phone turned off and wont turn back on or charge.

I came home from class a few months ago and my phone was flashing a light (it was either orange or blue I can't remember) so I left it to sit. I came back an hour later and it was dead so I plugged it in but there was no indication that it was charging. I tried different cables but that wasn't the problem. I tried every troubleshooting option and spoke to many technicians who couldn't work on my phone because its a "duos" that I bought overseas. I went back to where I bought the phone and they said they could fix it but they would need to wipe the phone. But I need all the information on it.

So I decided to fix it myself.

How can I diagnose if the battery is the problem or its something else?