In my world i clicked on the forget this world button to see what it does and this wasn't a good idea...
Is there any way you can undo that? I alrady tried reinstalling the mod and deleted the modfolder in the .minecraft path.
I hope that anyone can help me.

Unfortunately, once you've pressed and held the forget world button all item locations are forgotten and can not be undone; you'll need to "re-learn" them again.

how does this work? Beacause as i installed the mod and went into my world all items were already found by the mod is this possible that the mod does this again?

The mod remembers what items you've seen and where, so if you've started opening chests again it will remember where those items were and start filling the list. Other than that, I'm not sure, maybe the file stayed afterwards for whatever reason.

Okay thanks for your answer i try that.