WhatsApp is having some issues and is being reported as "down" by users across the world. WhatsApp is a service that has a pretty good up-time with very few problems, much like Facebook is nowadays. Right now, however, users from all over the world are reporting issues in sending messages to their friends.
Some users are also complaining about being unable to load conversations.
Downdetector users have flagged issues all across Europe, the United States, and South America. Reports surged over the past hour, it seems, so this is a rather fresh problem.
This may very well be a problem with WhatsApp's servers and it remains to be seen how quick they will fix things. Until then, however, Twitter is going crazy over this, with #WhatsApp already trending.