ml350 wont start

thanks for you answer, so we charged the battery , we had to have it towed to a mechanic and they said they checked all modules and found nothing, they THINK it is the key i have had a dead key for almost 2 years, my key stop working to lock and unlock about 2years ago i always just thought it was the battery , but now i had to pay 380 to get a new key ordered and that may not even be the problem. idk at this point