Have you recently installed Apple Music, Apple TV, or Apple Devices? If so they prevent iTunes working with anything but audiobooks and podcasts. Remove all three and things should return to normal.
It is possible that iTunes is trying to show a modal dialog box, which should always be at the front, but it is hidden behind the main window, which you then won't be able to interact with. You can try alt-tab to switch windows, or ctrl+alt+del to launch the task manager so you can end the iTunes task. Or you can reboot your computer.
If content is still missing from the iTunes library once you are able to open it normally, and select Music from the media selection tool at the top left, see Empty/corrupt iTunes library after upgrade/crash - Apple Community.
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