If the phone won't charge because it's been powered off for an extended period of time, the battery has died. Lithium iON batteries if discharged for extended periods tend to die.
If you don't have a backup on iCloud or in iTunes, you're likely out of luck. You'd need to have the battery replaced in the phone (which will only cost $49), but in replacing the battery, your data will more than likely be lost on the phone. You'd need to back it up before getting the battery replaced and since you can't charge it to turn it on, that's not an option for you.