If you were trying to send a message and you were blocked, it's because you sent to many friend requests where the person you were trying to 'friend' either reported you or ignored the friend request. Facebook is a place for people who already know each other to communicate. Facebook is not a place to make new friends according to Facebook. In reality, Facebook is a place to make new friends and this block serves to simply limit the amount of friend requests. This limitation is made to prevent spammers and protect low tolerance and 'fragile hearts' of female Facebook users from the persistent

Solution

This problem isn't a trivial one. Many Facebook users, including myself, do so for business. Many Facebook accounts are connected to Facebook Pages, and this block affects all connected accounts. If you wish to avoid this block, only add 10 people a day max. Don't message people outside of your friends list more than 15 times a day.