Russia will cut off US access to the International Space Station

Symbolic move only.

The USA wants to operate aboard ISS until 2024. They're currently contracted to use Soyuz for astronaut transfers until 2020. Russia is refusing to renew the contract past 2020.

However, Dragon version 2.0 should be flying by 2015 and crew-rated by 2017, and Boeing's CST-100 is due to fly unmanned in 2017 and possibly manned before the end of that year.

So the USA will have alternative ways of sending astronauts up to the ISS before the Russian embargo kicks in.