Windows.applicationmodel.dll is a dynamic-link library file that is a part of the Windows operating system. It contains functions and resources that are used by various Windows applications and services. It is critical to the proper functioning of the Windows operating system and many of its built-in apps.
Deleting this file will cause the Windows operating system to not function properly because the file is needed for the operating system and many of its built-in apps to work correctly. The absence of this file will prevent the operating system from booting up and will also cause many errors and crashes in Windows apps. Your only option in this instance is to restore from backup or perform a re-install.
Reinstalling Windows or restoring from an image backup is typically the only way to restore the file if it has been deleted.