After launching a nuclear strike, a player will find that for a period of time they can neither buy nor sell at the market, due to the fact that other nations are upset that the player's nation has used nuclear weapons. It lasts around two minutes, and cannot be reduced in time. It can however, be prevented if and only if the player has the Space Program wonder. A nuclear embargo lasts for two minutes if one nuclear device is detonated, and with each consecutive explosion two more minutes are added.
Most of the time, nuclear embargoes don't really matter. By the time nuclear weapons become available, most nations have a robust economy and little need to buy or sell resources.