How Can I Reinstate the A key on My BlackBerry as the Keypad Lock?

I have had my BlackBerry for 2 years now. All this time, I have been able to press the A key for the keypad to lock. A couple of days ago when I did this I got the message, “Assign a speed dial to the A key?” I’m guessing that something must have been pressed accidentally that caused the A key to change its function. I have not uploaded any new software or updates. So, any ideas on how to get my A key to be the keypad lock once more?