I suppose, the Find String List window is docked at bottom of UltraEdit main application window with a height near 0. A useful height of the window can be restored by clicking with primary (left) mouse button on small line of title bar of nearly not visible window, hold primary mouse button, drag mouse pointer upwards to increase height of the window and leave the primary mouse button. Please see the attached image showing at bottom the main application window of UltraEdit with Find String List window with minimum height (just thin small line of title bar) and at top after increasing the height how it usually looks like with a useful height docked at bottom of main application window.

Find String List window with useful height and with by mistake minimum height docked at bottom of main application window.

By default the directory %APPDATA%\IDMComp\UltraEdit contains the workspace file uedit32.in0 or uedit64.in0 depending on using 32-bit or 64-bit UltraEdit. Update: The used workspace file is uedit32u.in0 or uedit64u.in0 since UltraEdit for Windows ≥ v24.00 because of being Unicode encoded with UTF-16 LE instead of ANSI encoded.

This file contains for the Find String List view the sections [Dockbars.002BasePane-33424] (visibility control) and [Dockbars.002Pane-33424] (floating or docked, auto-hide on/off, window position and window width/height for docked and floating window state).

While UltraEdit is not running open with Notepad the workspace file and delete entire section [Dockbars.002Pane-33424] because it looks like the window is floating below another window or is positioned on an extended screen, but there is currently no second screen turned on.

Save the workspace file, exit Notepad, start UltraEdit, run a Find with List lines containing string and the window should appear docked at bottom of the main window with a height of just 5 pixels. The height must be increased to see the window title and the found lines.

Instead of deleting the entire section [Dockbars.002Pane-33424] it can be also replaced with the following block:
This results with IsVisible=1 in section [Dockbars.002Pane-33424] in getting the window displayed after next start of UltraEdit docked at bottom with a useful height and floating state is also on primary screen with a useful height and width.