Until last week, I was able to switch users by accessing the start screen, clicking on the picture, and then selecting the user name I wanted to switch to. This no longer works: no option to switch users appears in the drop down menu.

Additionally, I am now forced to user "Ctrl+Alt+Del" to access the login screen.

Is this due to an update that I have installed? Is it a setting that has been changed?

Which Windows update were installed recently installed prior to the issue? Please check the Windows Update history to find out the update that were installed.

To check which updates are installed so far, open Control panel by pressing Windows key and X key and select Control panel. In the search box, type view update history.

Provide us the KB number of the update that have installed prior to the issue.

To disable the Secure sign-in in Windows 8, please do the following.
- Press Windows key and the R key to open the Run prompt.
- Type NETPLWIZ and hit enter.
- Click on the Advanced tab and remove the check mark from "Require users to press CTRL+Alt+Delete"

If you are using Windows 8 Pro edition, then you may try these steps.

To enable the Fast User Switching, do the following.
- In the Search Charms box, type GPEDIT.MSC and press enter.
- This will open Local Group Policy Editor. Now, go to the following location and look for Logon in the left pane.

Local Computer Policy/Computer Configuration/Administrative Templates/System/Logon

- Just double-click Hide Entry Points for Fast User Switching to bring up a dialog box to change Fast User Switching policy setting.
- Click Disabled and press okay.

Once done, you will have to enforce the change made to Fast User Switching policy setting. To do so, close the Local Group Policy Editor and open the Run dialog box (Press Windows key and R key). Enter the command “gpupdate /force“ and click OK. The policy will be updated and applied on all user accounts.