How do you open the inventory menu in Undertale?
While playing Undertale, you'll want to use C or Ctrl to open your menu and Z or Enter to select
When you start up the game, the title screen features an option menu where you can reconfigure your keys. (or test the game with a joystick/gamepad if you want to use something other than a keyboard.)
In your case, it sounds like the controls might have been reconfigured at some point.