Phone died and won’t turn after charging for hours

I was given this phone by my grandmother (who got a new phone as a gift), and when I first got it, I didn’t do much with it since an iCloud password was needed to reset, and I don’t know what it is. It stood on for a few days (3 or 4) before it finally died, and when I left it charging for a while, it didn’t turn on at all. I left ir charging for a few more hours, and after that didn’t work, I tried multiple chargers and outlets in my house, yet nothing happened. I held down the power button and not even the battery screen shows. I believe it may be a battery issue after reading stuff online, but I would be grateful if anyone could help me figure this out.