https://stackoverflow.com/questions/46632070/notepad-search-window-disappeared
The issue seems happens if I change monitors configuration , e.g. if I worked on laptop with 2 attached monitors, and next time open Notepad++ when having single laptop monitor.
For me it was sufficient to delete %appdata%\Notepad++\config.xml (not the whole folder).
Close Notepad++
Open %appdata%\Notepad++ folder (e.g. C:\Users\{UserName}\AppData\Notepad++ )
Delete (or rename in case if you want a backup) config.xml
Start Notepad++