It says I got banned for spam, but I don’t know how this happened. How do I get myself back on the forums?
You could try building up some Karma here, then asking a mod on the forums to let you back in.
Karma indicates the community trusts you and values your contributions. With 0 karma the question will inevitably be asked ‘Did you deserve your ban?’
If it’s just a misunderstanding, I would just contact support: http://unity3d.com/contact/get-in-touch?type=info