@musya - Since you have likely closed the board since this post, you won’t be able to “undo” the changes using Ctr/Cmd-Z (and if you haven’t closed the board, give that a try!).

Board history: Versions

If you were on a paid or Education Plan, you likely could have used the Board history: Versions feature to restore a previous version of the board; before you made the changes. And, unfortunately, even if you upgraded to a paid plan – even if just for one month – previous versions of the board are not captured when you are on a Free Plan team:

Source → https://help.miro.com/hc/en-us/articles/360021668819

Filtering out objects

At this point, you will need to manually fix what was done. One feature that might ease the pain is selected-objects Filter. Once you click-and-drag around everyone (or Ctrl/Cmd-A to select all), you could Filter an object type that you wish to remove from the mix of objects, e.g., the object that there is the most of. For example:

@Robert Johnson thank you a lot, Robert. 

I was thinking the same  that paid possibilities will start to work from the date I paid and it will not help me to restore. Yes, I already closed the board.

Thank you for explaining how to filter out the objects.

@musya - My best guess is that you clicked on the Auto Layout tool, but that action should still be “undoable”.