I often find my iTunes has reverted to the default location for the iTunes library file (my documentsmymusiciTunes) even though the option under preferences, Advanced for media file location is the one you want.
To redirect again there appears to be no option, but the easy way is to:
Select iTunes icon on desktop, press shift and keep it pressed, press enter to open iTunes (don’t double click mouse as this will open multiple items i.e. sees shift as a multiple select option).
You will then be prompted to choose a library or create a new one.