OXT Alt Search Replace was installed. Now the menu Edit/Find and Cmd-F are disabled.
Who did this?
How can Cmd-F functionality be restored?
Alt Search & Replace does not interfere with keyboard shortcuts, so the problem should lie somewhere else. Nonetheless, Tools → Customize → Keyboard, and assign whatever you want to whatever shortcut you want.
In LibreOffice for Windows, Ctrl+F is linked to the Find box in the Search Toolbar, Ctrl+H is linked to the Find & Replace window. So you should try Cmd+H for that window, or customize your keyboard shortcuts.
If this answer helped you, please vote it with 
(here on the left). That will help other people with the same question.