If the Yellowstone supervolcano erupts, it would blanket the entire United Sates in ash, disrupt crop production, cause traffic accidents, cause respiratory problems and result in climate change. Ash alone has serious side effects, such as collapsing buildings.
If Yellowstone erupts, the states nearest to it would accumulate several inches of ash, with the states further away also getting smaller amounts. While the weight of inches of ash can collapse buildings, smaller amounts can become a serious obstacle on roadways, causing accidents and shorting out power for millions of residents. Ash can also cause many negative health affects, including respiratory burns and eye and skin abrasions.
Another serious consequence of an eruption would be its effect on crops. The ash would make it impossible to grow crops or keep livestock alive. The sulfur in the eruption would cause climate change for not only the United States but the entire world.
However, eruptions of this scale are rare. The last one occurred nearly 70,000 years ago. The probability of another one soon is rare, almost one in a million. If there is volcanic activity in Yellowstone, smaller eruptions are more likely, which have few effects on anything but the immediate surrounding area.