You can find out if someone's blocked you on Yahoo! Messenger, or if the user is simply online or otherwise not available to chat. Yahoo! doesn't send you any sort of notification when someone blocks you, so it's up to you to determine whether or not you are blocked from communicating with a specific person. Avoid jumping to conclusions about potential causes for a block until you're absolutely certain the other person has blocked you.
Sign in to your Yahoo! account and wait for the contact list to appear. Scroll through the list and look for the name of the contact you believe has blocked you. If a yellow smiling face appears next to his name, it means he is online and hasn't blocked you.
Double-click the person's name if the interface indicates that he is offline. Type her a message and hit "Enter." If you receive an error message in return, it means that she is offline and may have blocked you. If you don't, it means she is invisible -- she is actively using Yahoo! Messenger, but has chosen not to show herself as being online. She may or may not respond to you.
Ask another of your Yahoo! contacts who is also friends with the person you believed blocked you if he appears online in her list. If he appears online in her list but not in yours, he has blocked you. If not, he is simply offline and not using Yahoo! Messenger.
Contact the person who blocked you using a non-Yahoo! means if you believe the block occurred in error or that you can resolve the conflict that led to it. Avoid creating a second Yahoo! screen name with the intent of contacting or harassing the person, as this may get you permanently banned from Yahoo!.