Since you have merged changes from trunk to branch and want the branch back in previous state in which it was before merge you can do the following :
- Use SVN log command and check the revision no before the merge happened .
- Do reverse merging for getting the previous state " svn merge -r HEAD:stable-revision-branch . "
- Do svn commit .
Please refer the link for more clarification SVN reverse merge?
Regards
Jyotsna