I can´t access the admin-menu of the forum anymore. And the buttons for "new topic" and "reply" are also gone. Why?
All I made were some changes on the forums-header. Is this the reason?
Yes, this seems to be the reason why.
Please read the FAQ how to change some values of the header. There is a safety-function to protect the link "... powered by eBoard" being removed out of the header.
Also if the link wasn´t removed, if you changed too much of the format of the source-code, the forum will disable itself.
Reactivation is only possible with a new install or from the admin-team.