Command key malfunction

The Command Key does not function. In the applications that I use—Mail, Pages, Sibelius (a music program) pressing Command plus a letter merely results in the letter itself. For example, Command plus C should result in Copy, but only brings the letter C. I must have inadvertently changed something in the Keyboard section of System Preferences. I have spent several hours trying to sort this out, without success. Can anyone help me, please? Anticipatory thanks and gratitude, from

Eugene

[Personal Information Edited by Moderator]