You can try this:
1) Exit from your user login
2) press CTRL+ALT+F1 to get to a console
3) login as your user
4) ls .config/menus
4a) if this directory does not exist, create it (mkdir -p ./config/menus
4b) move any files in that directory to a temporary holding spot (mkdir temp; mv .config/menus temp
5) move any previous application link files (mv .kde/share/applnk* temp
6) exit and press ALT+F7 to get back into GUI mode
This basically creates a "blank-slate" for your customized menus. This is especially useful if you've done custom KDE menus in previous versions and have kept them around through upgrades.
If you do have previous menu entries, you can look at the files you backed up to create new menu entries. The actual menu entries end up getting stored in .local/share/applications
and sub-menus are in .local/share/desktop-directories
Note that the .config/menus/applications-kmenuedit.menu
file is an XML-based file, while the sub-menu and menu entries are "quasi-ini" style. Be careful if you modify any of these files by hand...