The following applies to Windows 10

For reasons that are unclear, the Windows Update engine occasionally will fail to load and update services will be unavailable. When this happens, it is necessary to re-register the Windows Update engine's dynamic linked library files.

Remember to log on to a user account with administrative privileges (firewall or antivirus) and disable any security software before attempting the fix below.

Click Start > Run to open the Run dialog box. Type the first command listed below and then press enter. When registration is successful, repeat these steps continuing to the next command until all have been entered and successfully registered.

A reboot should not be required. Try Windows Update again to see if functionality has returned.