Probably because you broke one of the rules or you're constantly breaking the forum rules. See it here: https://bitcointalk.org/index.php?topic=703657.0. Also, try to avoid plagiarism. That's the usual offense people make that'll lead to banning of account.

Depends on how serious your offense is. Sometimes the duration of ban is stated, otherwise try to pm a mod and explain your side. But be cautious, mods may think you're evading your ban through another account.