Russia has been flexing its muscles recently, "allowing" the Crimean Peninsula to rejoin it after the Ukrainian revolution. Understandably, an expansionist Russia ruffles a few feathers here in the western world, and so the U.S. and EU implemented a series of sanctions. Russia being a bit of a heavyweight in the world of international relations itself, the sanctions didn't go unanswered, and NASA is now barred from using Russia's Soyuz spacecraft.
It's worth noting that the sanctions imposed on Russia are so severe that to find their historical equal you'd have to turn the clock back to the days of the Soviet Union and the Cold War. They even include an export ban on high-tech rocket engines used to reach orbit. Russia's credit outlook has already been downgraded and the nation is under threat of slipping into recession.
That said, the International Space Station (ISS) is a symbol of cooperation and humanity's progress toward the peace and prosperity of the final frontier. In the past, political disputes between the U.S. and Russia have kept the dream of that peaceful tomorrow intact.
With the Soyuz spacecraft off-limits to U.S. astronauts, we currently have no other means of getting our people up to the ISS. This comes at a particularly bad time for the aging ISS itself, which NASA would like to retro-fit to endure until the year 2024 — four years past its expiration date. The U.S. will also have a bit of trouble getting satellites into orbit without the Soyuz, not to mention the other negatives that come with tensions with Russia.
Here's hoping everybody calms down ASAP and international relations return to those higher morals which prosper in Star Trek and other utopian sci-fi.