Also, we've used up a good deal of what got us started the first time. Unless the metal ores were returned to their starting places in the ground when the technology disappeared, the quantities reachable and usable without high technology simply aren't there anymore--a chicken and egg problem. We'd also have far less runway to wean ourselves off fossil fuels this time; coal would be useful, but whale oil would be a more workable solution than petroleum. So even if when technology disappeared we lost the need for food and water, we still might never make it back.