Some other program on your computer is likely capturing the shortcut. For example, some users have recently reported that their AMD/ATI video card software is stealing the cloze deletion shortcut key. When they disable the shortcut in that program's preferences, the key starts working in Anki again.
If you run into similar problems, please try temporarily closing/turning off other software running on your machine, until you discover the cause.
If you use add-ons in Anki, please also try disabling them to rule them out as the cause of the problem.