How to restore merged branch after it has been reverted

So … I have merged a branch and noticed something went wrong (probably kept a change I should not have). Consequently I reverted said branch (aka restored an earlier version in the history of my master file).

How do I restore my "merged" branch to try and merge it again?

Online it said, that this is impossible. Are all the changes completely lost now? In my case those are luckily small and I can redo them rather quickly, however I do find it a bit worrying …

Should I ever have some big changes in a branch it would mean completely redoing them if I am not mistaken? This does not really build trust in the system if I am being very honest 😅