Yes, the Bitcoin Network has been down two times in the past. Once in 2010 for around 9 hours, known as the Value overflow incident where a block contained a transaction that created 184 billion Bitcoins for three different addresses. The network was soft forked to not allow any more than 21 million Bitcoins ever.
And once again in 2013 for 7 hours, known as the March 2013 Chain Fork, where a block that had a larger number of total transaction inputs than previously seen was mined and broadcasted. While Bitcoin 0.8 nodes were able to handle this, some pre-0.8 Bitcoin nodes rejected it, causing an unexpected fork of the blockchain. A new released a version 0.8.1, forked directly from 0.8.0 solved this issue without any harm to the network.
Depending on whether you call these incidences a network outage or not Bitcoin would have an uptime of 99.99% since its creation as of the time of this writing, and 100% uptime since 2013, meaning the Bitcoin network has never been down or unavailable since 2013. These levels are unmatched even by the largest tech companies in the world.