The Russian economy is already suffering from weak growth, partly due to the Ukrainian crisis and partly due to sanctions by Western countries. With oil revenue accounting for about 45 percent of Russia's budget, and the fact the 2015 Russian assume $100-per-barrel oil prices the government will have to either draw down from its $74 billion foreign-exchange reserves or cut back on planned spending.
The Russian economy has not been doing well for a while, and could collapse through a combination of souring relations with the EU, the crisis in Ukraine, its human rights issues, and falling oil prices. While allies may have wanted to help Russia in the past if its economy faltered, its current relationships with allies could make the oil prices hurt Russia far worse than they would have in the recent past.
I do not believe the Russian economy will crash from falling oil prices. Although the oil business is one of the most important economic resources in Russia, I think they will survive if they focus their efforts on new ways to make money and make up the difference from the decrease in price.
Russia will not economically crash. This is because it has plenty of other industries besides the oil industry. They have food and mineral exports that also hold up their economy. Also, down spirals in prices do not last that long. They will have to weather a small storm, and then it will all be fine again for Russia.