Symptoms - when you attempt to add a new user account, or edit an existing one, the checkboxes to select whether they are a sysadmin are no longer present.
You're using Microsoft Internet Explorer v9, right? Try this -
1. In your browser window, press F12 on the keyboard. This opens the "developer options" at the bottom of the page.
2. Refer to the screen shot below - if your window mentions "Compatibility View", click on that wording and change it to "Internet Explorer v9" (that is, ensure that the Compatibility View option is not selected).
That should do the trick!