Russian officials say the country's most popular social network, VKontakte, was mistakenly put on a blacklist of banned websites, but the error has now been fixed.
In some parts of Russia, including St Petersburg, the mistake immediately blocked access to the site on Friday.
But the state communications regulator says access has now been restored.
VKontakte has 200 million registered users. Last year it was used heavily by opponents of President Vladimir Putin.
A spokesman for the state regulator Roskomnadzor said Friday's incident was "a human error".
Last November a controversial law took effect which blacklists websites considered harmful to children.
The law allows the government to take sites offline without a trial.
The authorities say the goal is to protect minors from websites featuring sexual abuse of children, offering details about how to commit suicide, encouraging users to take drugs and sites that solicit children for pornography.
Critics see it as another attempt by President Putin to exercise control over the population and stifle opposition.