Suddenly phone is dead for no reason (it has non-removable battery)

We have a Micromax Q380 at home which is running Lollipop. It was working fine last night and suddenly it is not responding this morning. Power button and it's combination with volume buttons won't work. It just looks like a dead phone. Display doesn't light up at all, even when charging. It's battery is non-removable which is the sad part of the story. What can I try before handing it over to a service store?