This is likely a duplicate of Buttons are missing in the file chooser dialog. What should I do?
You could try the following command to reset the preferences that have likely gotten messed up:
gsettings reset org.gnome.settings-daemon.plugins.xsettings overrides
In summary, elementary overrides some GNOME settings that place the buttons in the Save/Open dialog at the top of the window where the path bar is and moves them to the bottom. If this setting accidentally gets undone, there is nowhere for the buttons to go at the top of the window so they simply aren't displayed.