My old iPhone 5c won't turn on even though it was working fine a month ago and has just been in a drawer since. How can I get it working?

I upgraded my iPhone a month ago and have been experiencing problems so I need to get it fixed. It will be weeks until I get it back so I decided I d use my old iPhone 5c until then. But I can't turn it on, it was working fine last time I used it a mo th ago. I tried charging for hours and hours and holding sleep and home buttons etc and it still won't come on. My charger is fine. How can I turn it on?

Charge the battery.