I kept trying to turn on the phone with the power button, the galaxy logo appeared and disappeard, then the grey "it's charging" battery icon appeared again as if it still needed to charge...but it says 100%!!!
Welcome to Android Central! Which phone is it, and how old is the battery? The battery might be unable to hold a charge, even if it's registering a charge. Try a new battery.
Or keep the charger plugged in after it reaches 100%, then hold the power button down until the phone starts. If it still doesn't start that way, the battery is probably beyond dead.