Is it safe to charge an iphone for more than 12 hours if it won't turn on?

An iphone 6 hasn't been used in a few months. It's been in a room that has been a little cold in the last month. I think it's battery is completely dead. I tried charging it overnight and holding the power and home button but nothing happened. Is it dangerous to charge it during the day too? I need an answer as soon as possible. What should i do?

No it will make damages to your battery and cpu

Take it to an iPhone repair specialist.