Customize menu > Revert to Startup Layout
Revert to Startup layout automatically loads _startup.ui, which returns the user interface to its startup settings. This temporary system file is created automatically when you start the program. Use this option to return the UI to startup settings.
See Load Custom UI Scheme for information on how to load different UI schemes on startup.
If you have Save UI Configuration On Exit turned on in the General panel of the Preferences dialog, the current UI file is overwritten when you exit the program.