Facebook permits users to communicate freely with one another, but also allows individuals to limit access to their profiles on an as-needed basis. Namely, if another Facebook user decides he or she doesn't want you to view his or her profile and uploads or contact him or her, that user blocks you. You can take any of several courses of action in the event someone blocks you.
What Blocking Entails
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Blocking someone on Facebook completely prohibits that person from contacting you -- or accessing any of your content -- on Facebook. For example, if someone you've blocked searches your name, it will not appear in results. If that user attempts to access your Facebook URL directly, that user will receive an error message. This being said, Facebook doesn't give users explicit notification of having been blocked. If you believe someone has blocked you, try visiting that person's profile or searching by name.
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Other Possibilities
Before you take action to contact or question someone who's blocked you, you need to make sure that person's actually done it. If you can't access a person's profile or find him or her in search results, it's also possible that user's deleted his or her profile altogether. To confirm or deny this, ask a mutual friend to attempt viewing the person's Facebook profile. If the friend also can't see the profile, it's most likely the person in question has deleted his or her Facebook profile.
Direct Contact
If you've been blocked because of a personal or professional disagreement with someone, the best way to resolves your Facebook woes is similar to what would be most effective offline: contact him or her directly. Since you can't contact via Facebook, send an email or make a phone call. Be concise and specific in your questioning and ask what steps you can take to restore your Facebook friendship -- and if your real-life friendship has also been compromised, that relationship as well.
Things to Avoid
When someone gets blocked from another person on Facebook, one action that could be taken is creating another profile as a means to contact that person and view his or her Facebook profile again. Whatever you do, avoid involving Facebook in your dispute resolution process, which could result in your privileges being suspended or revoked altogether. Additionally, if time passes and you don't receive a response, don't contact that person repeatedly. Harassing someone who's already made a decision does little more than aggravate.