Windows 8.0 users need to make certain that they have been able to update to 'Windows 8.1 Update' as Microsoft have stated:
"You must install Windows 8.1 Update to ensure that your computer can continue to receive future Windows Updates, including security updates."
"For our consumer customers, the Windows 8.1 Update is a required update to keep Windows 8.1 devices current. It will need to be installed to receive new updates from Windows Update starting on May 13th. "
For a detailed explanation, you can read:
Given negative reception that Windows 8.0 received and the significant improvements Microsoft have made in subsequent updates to arrive at Windows 8.1 Update, I doubt many Windows 8 users will willing want to stay on Windows 8.0. However, there have been technical difficulties reported by many when attempting this update, with some recalcitrant devices stubbornly remaining stuck on Windows 8.0.
Please make sure that you are not caught in this situation. You may need to seek technical help to force the update so your device can remain as secure as possible.