Most blocks are set by the admins of one community, and only stop you from editing or chatting on that one wiki. The name of the blocking admin should be on the block notice you see when you try to edit there.
You may be able to contact the blocking admin by leaving a message on your message wall. This is usually still open to blocked users and changes will usually be seen by others in the community. Your message wall can be found under the profile menu at the top right of any page. You can also contact the admin on their wall on Community Central.
However, if these options are not possible in your situation, then the best advice we can give you is to wait for the block to expire or join one of the hundreds of thousands other Fandom communities available.
If your block is global, please contact us.