I have done this in some detail in the past (in ASB IIRC), but the U.S. exports somewhere in the area of 43% of the world's coarse grain, or did the year I did the original research (the figure varies year to year depending on grain yield).
While there are some back up sources, notably Canada, Argentina and Australia, they lack the capability to expand significantly. IMO what you get is severe famine in Africa, especially East Africa, where crop yields have never recovered from the 1980s, followed by serious issues in Pakistan. What is likely to happen is that the price of grain would skyrocket (with meat prices going flat out of sight), so the wealthier countries would be able to survive, albeit in a really screwed up global economy. Poor countries, especially those that rely on UN and NGO food shipments, will be badly damaged.
Any potential death toll depends on how long the loss of exports lasts. It takes a while to starve to death, assuming any food is available. It also very much depends on if it something limited to the U.S. (this is very unlkely, anything that takes the American crops out of the picture will almost certainly take out Canada's fields as well). If it is open ended, and includes Canada, 500M is not an unreasonable middle ground, with 1B being the utter worst case (these figures assume that North America can still feed itself, even if nothing is left to export).