Your Command key is stuck programmatically by e.g. running
key down command in AppleScript.
If you didn't program, try pressing it repeatedly, and make sure the keyboard is clean.
Try to log out and back in. This will reset the relevant UI state.
If you really programmed it in AppleScript, prepare by create a Service in Automator that receives no input in any application and has the following Run AppleScript action:
tell application "System Events"
key up command
key up option
key up control
key up shift
This will reset all modifier keys' states. Save to make it available in any application's Service submenu of the application menu.