Certain people can be barred from re-entering the US for various reasons – overstaying their visa for at least six months, multiple criminal convictions, etc. If such a bar is placed on you, you are in a difficult situation – if you are outside the US you will find yourself unable to return, while if you are inside the US you will find yourself unable to leave without facing a bar on re-entering the US, even if you are issued an immigration visa at a US embassy overseas.