MSN Messenger, formerly known as Windows Live Messenger, was a widely-used instant messaging service that allowed users to communicate with each other through text, voice, and video calls. However, it was possible for users to block other users from contacting them, which could lead to confusion if someone was unsure whether or not they had been blocked.
There were several ways to check if someone had blocked you on MSN Messenger. One way was to try sending them a message. If the message was not delivered, and you received a notification that the user was offline or unavailable, it was possible that you had been blocked.