Two weeks ago, the Russian Federation announced that Russian visas are required for entry onto the territory of Crimea.
This requirement does not apply to citizens of:
1. Russia (of course)
2. other CIS countries (including Ukraine)
3. countries that have a visa-free regime with Russia, such as Argentina
Probably, there weren't many non-exempt foreign nationals in Crimea when the announcement was made, but such persons must apply to Russia's Foreign Ministry office in Simferopol, whether they wish to remain in Crimea, or to exit (though citizens of some countries, including the US, don't need a visa to exit from Russian territory).