Hi guys. I am only a few days old with Adobe Photoshop. Recently I've encountered a kinda lame problem. I couldn't use keyboard shortcuts like I used to. I could undo (Ctrl + Z), fill background color(Alt + Backspace), and etc. But I can't do any of these now. I mean it's not just these 2.

I asked a friend for help and he told me to go Edit -> Keyboard Shortcuts. The shortcuts were there, but I just couldn't use them.

Can anyone help me out? Thanks a lot, because I can't use Photoshop with a peace of mind right now. Thanks a lot guys!