Itunes app is missing

If you mean the iTunes icon on your Windows taskbar, then there was a recent update to iTunes which will have rendered the taskbar shortcut ineffective. Delete it and add a new shortcut.

If the iTunes icon still appears on your desktop, simply drag it to the Taskbar.

If the Desktop icon is missing as well, select the Windows key and find the iTunes programme (app) listed in the All Programmes menu (it will be listed under the iTunes folder). Left-click the iTunes programme and drag it to the Desktop. If you want it on the Taskbar as well, drag it from the Desktop to the Taskbar.

Is this a new Windows computer? If so, iTunes is not pre-intalled.

Did you install Apple Music for Windows, the Apple TV App or the Devices app? If so, any of them will sort of disable iTunes.

It will still open, but will no longer offer access to your iTunes library.