Update 3/13/2020: All Battle.net services have been restored and players should once again be able to access their Blizzard Entertainment games. This may require a restart of the Battle.net games launcher, however. Original story follows.
On Friday, March 13, the Battle.net servers went down for many of those playing Blizzard games. This means that players are currently unable to play titles ranging from World of Warcraft to Diablo 3, and right now, there is no estimated time when the Battle.net services will be restored. The problem has been going on for at least a few hours at the time of this writing, though it's unclear what caused it.
It's also unclear exactly how widespread this issue is. Reports of Battle.net being down on March 13 spiked within the last hour, so it seems it's starting to become a more widespread issue. The exact nature of the problem is also not entirely nailed down at this time. Those who were already logged in to Battle.net may still be able to access Blizzard's games, but those trying to log in are currently unable to do so.
While Blizzard fans may be the most impacted by Battle.net being down, it's worth pointing out that the launcher hosts other games as well. Perhaps most importantly, players play the hugely popular Call of Duty: Modern Warfare through Battle.net. This means that while Battle.net is down, Call of Duty fans on PC may also be unable to play the game or the Warzone battle royale mode.
Battle.net outages happen occasionally and are usually resolved within a few hours. The same is true for most outages when it comes to online video game services, whether it be Battle.net, PlayStation Network, Xbox Live, or whatever else.
Sometimes there are multiple outages for a one service in a single month, however. For example, EA servers went down multiple times in February, with a couple of occasions lasting much longer than just a few hours. Last year, there was also a string of times when Xbox Live went down multiple times from January to February.
We will update this post once Battle.net services have been fully restored and players are once again able to access their library of Blizzard games.